Move read_table() and Counter() to a top-level utils directory as they are used both by pcigale and pcigale-plots.

Create a pcigale/utils directory to store various utility functions in preparation for the inclusion of cosmology utility functions. Move pcigale/utils.py to pcigale/utils/io.py, which contains only read_table() and adapt the rest of the code accordingly.

We do not store the time grid in the SED anymore given that we assume it starts at 0 Myr with steps of 1 Myr, we can easily reconstruct to save it if needed. It should save a little bit of memory and it should go a little bit faster.

The name creation_modules is a bit vague. For more clarity and consistency we rather use sed_modules now

Specify the types and the constraints of the input parameters for the SED creation modules and the analysis modules. Once validation is activated, the validation module will be able to infer the constraints from the specified information.

Modules were a bit inconsistent how the input parameters are used. It was not always ensured they were the right type, they were sometimes read several times, and finally they were not always obtained from the _init_code() method. This is an attempt to clean that up as much as possible.

Implement an _init_code function so the initialisation of the module is separated from the processing of the SED. Because the initialisation can be slow, this should yield a speed-up when applying the module on different SED.

Clarify the description of the module parameters to ensure that the time grid is correctly formatted and add a test to ensure this.

Be more resilient by converting the array of SFR to float. This avoids a possible crash down the road when doing an in-place assignment of an integer with a float.

In the sfhfromfile module, an extraneous offset in the column index made that it took the previous column as the SFR rather than the selected column.

The galaxy_mass parameter confuses a lot of people. Change its name to sfh.integrated because it is actually the integral of the SFR.

By construction the SFH in cigale is a comb. Every 1 Myr stars are formed. All at the same time. In effect star formation is not continuous. This means that we should not use trapz to integrate the SFR or use an analytic relation. Rather we just sum, which is correct for the discrete nature of the SFH.

When computing the galaxy mass, convert from yr to Myr outside of the integral. That way we only multiply a scalar by 1e6 rather than a large array. Good for speed and memory bandwidth.

Switch back to ordered dictionaries for the input parameters after all. That way the list of parameters is kept into a logical order when building the configuration file. The impact on performance is negligible.

We rely on OrderedDict to keep the order of the parameters. However given the way the code is structured currently, there is really no reason for this. So let's just switch to the standard dictionary. This has two advantages. First the standard dictionary is much faster than OrderedDict (implemented in C rather than in pure python). Then, standard dictionaries are marshalable, which makes them usable as keys for other dictionaries.

The "galaxy_mass" parameter must not be set to 1.0 in the stellar
emission modules because when using CIGALE to generate SED without
normalising the 1.0 is wrong and misleading.

Instead, we compute the real galaxy mass (the integration of the SFH) in
the star formation history modules.

Switch SFH ID to the column number rather than the column name because for memory efficiency reasons all infos are float values.

Remove the possibility to use the same module several times. Simplification of the code ensues. Now sed.info keys are prefixed with the characteristic of the module (stellar, nebular, etc.).

Remove the author email addresses from the source files and add a AUTHORS.rst
file with the addresses and current affiliation to make it easier to update
the information.

Also, give copyright to the University of Cambridge and the University of
Crete for code developed by authors working there.

It will make the documentation clearer.

For the creation and analysis modules, move the contents of the
common.py files to the __init__.py.

Move the SED creation modules to pcigale.creation_modules to be more
consistent with the analysis module location.

In the sfhfromfile module, make the SFH normalisation to 1 solar mass
produced at the given age optional. When using pcigale to simulate SEDs,
one may not want this normalisation to happend.

Such SEDs should not be stored in the database (but SEDs made using
sfhfromfile should not anyway be stored in the database). We will have
to make the SED storing optional in the warehouse.

Instead of prefixing the key in the SED with the module name, we postfix
them with the same postfix as the module name if any. This makes easier
to inter-change modules adding the same information (for instance
attenuation modules adding the attenuation value).
That implies that when using several modules adding the same
information, one must postfix the name of each.

So that the configuration files and the serialized data have a logical
order.