When creating the SED of the best SED, select only parameters at a given redshift first as the index of the best χ² is computed on this subset.

Make model computations much faster. On a test run we spent 20% of the time just computing the luminosity distance. Now the information is just provided by the redshifting module. If the information is not present we just assume a distance of 10 pc.

Treat the total galaxy mass as a normal parameter. The special handling of galaxy_mass already got removed in the previous commit.

The initial implementation of the parallel analysis ended up grinding pcigale to a halt. This was due to the numerous array transfers between the main process and subprocesses. To solve this problem, we now share arrays through a module. This has the advantage
that thanks to copy-on-write, we almost never need to actually copy arrays. Now all the subprocessing run at full speed. Quite a few changes for a patch that only starting with the activation of cache clearing.

Implement partial cache clearing in order not to clog the memory with models that are not necessary anymore. This requires the proper computation of models that will not be used anymore. This is why we used ordered dictionaries to ensures that parameters are always considered in the same order.

If there is no redshift information, we assumed that the redshift is 0. Also correct a bogus test in the redshifting module.

Make pdf_analysis parallel. This required important changes. Functions saving data have been moved to the utils.py file. Workers for the parallel processes have been put in workers.py. To make sure we eliminate models incompatible with the age of the universe now the latter is indicated in sed.info for easy access. Ditto for the redshift.

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.