Add the necessary structures and methods to add and retrieve them from the
database.

The redshifting also lower the fluxes (that's different from the
cosmological dimming).
Also, simplify some methods.

The process method was used to complete and check the parameter
dictionary before launching the _process one defined in each module. Now
that the parameters are dealed with at instantiation, this separation is
no more useful.

Also, modify the attenuation modules to read the filter in the database
at instantiation.

In order to speed-up module re-use, get the module needs (ssp,
filters...) out of pcigale database at module instantiation instead of
doing this everytime "process" is called.

To do that:
- we must give the module parameters (and proceed with their checking)
at
  module instantiation.
- we must add the possibility for each module to have its own
  initialisation code.

In addition, to create the configuration file, we must be able to load
the modules without any parameter to query them for their parameter
list.

The bc03, dh2002 and m2005 modules were changed to use this new
behavior.

To avoid problems that might occur when, for instance, using an integer
instead of a float, we now cast the module parameters to their expected
type. This will also have the advantage to raise an error when a
parameter can't be interpreted with its expected type.

Also, use integer for age parameters as we are now using Myr as unit.

Thanks to Médéric Boquien.
The normalisation factor must take into account the error associated
with each filter flux.

Thanks to Médéric Boquien.

It's a dictionnary to hold the spectral lines associated with the SED.
We use a disctionnary to be able to differentiate several sets of lines
(e.g. lines from old and young populations).

TODO: Complete the code to add the line contributions to the integrated flux.

Rename lumin_contributions to luminosities and use a 2 axis numpy array
instead of a list of arrays. This should accelerate some operations.

The star formation history is now stored in its own property.
TODO: maybe automatically add age and instantaneous SFR to the info dictionnary when setting the SFH.

To perform the convolution with the SFH, the rate of ionizing  photons
must not be stored as the log of its value.

The best SED must pass through the redshift/IGM module before being
saved/plotted.

Using Gyr leads to some errors when comparing times (because they are no
really equals when they shoud). So we switch to the use of Myr as it is
the precision for our times.

Maybe later, we'll go further and use integers for these values.

Rename it to dustatt_calzleit in preparation of a power law module.

* Multiplicate the Calzetti-Leitherer continuum by a power law.
* Add a UV bump.
* The luminosity under 91.2 nm to 0.
* Extrapolate the continuum over 2200 nm until attenuation reach 0.

Oops, this should have been corrected a long time ago.

By default, an exception is raised when adding an existing key to the
info dictionary. Make it possible to bypass this, for instance to add
age if we want to combine various SFH.

If we redshift a SED using a module, we must not redshift the wavelength
to compute the λ, Fν array tuple. Thus we make it optional and not
default.

Convert the warehouse a its own module and use submodules for each
"back-end".
Also, make the back-end configurable with a parameter of the psum
analysis module.

It's better (for now) to use only get_from_cache and get None when the
object is not cached.