Move the plotting functions out of the analysis modules. Plots are now generated using an external script named pcigale-plots. This script is ready for multiprocessing but it is not yet activated. Finally, we can now save the reduced χ² values (which is required to plot them).

Tiny optimisation. Create arrays are empty rather than being zero-filled as we are going to assign all elements later on anyway.

Change the way redshifts are handled. Now all redshifts are specified in pcigale.ini. To compute the χ², we simply take the models corresponding to the closest χ².

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

Better think twice before pushing...

In fact, it would be better not to compare the wavelength grid sizes if
most modules change the grid *and* have a new grid of the same size
(that's not the case).

Not all contributions change the wavelengths grid (i.e. igm_attenuation). Optimize this case. It shaves off 1 ms/model.

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.).

glob is imported in the pcigale/session/configuration.py to allow its use in
the configuration file (with eval).

TODO: write a new configuration pattern to add several files with something
like "files *.fits".

Implement the nebular continuum emission. This has required some refactoring to have some homogeneous naming.