Allow pcigale to run without stellar populations so we can carry out SED modelling of dust only for instance.

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.

The quasar contributions must contain the name of the attenuation used
because using several attenuations will lead to a key error when adding
the second quasar contribution.

Class name should be CamelCased, arguments and variable inside functions
should be lower case.

There's no need to put my name in authors when I did not do anything.

Use the new post-fixed key names for contributions.