Slight update of the version number as CIGALE is now installable as non
editable.

The wheel is too big to fit in a Gitlab artefact.

Currently, a standard installation of CIGALE (`pip install .`) results
in an unusable installation as `setuptools.find_packages` does not find
PEP-420 packages.

This commit adds `pcigale/managers/__init__.py` to correct this.
Alternatively, we may want to switch
`setuptools.find_namespace_packages`.

Add pseudo filters to measure Halpha, Hbeta, and OIII. Beware that the fluxes will be affected by any underlying absorption line. Also make sure that the continuum part of the filter is not significantly affected by emission lines. It is recommended to adapt the filter depending on the actual resolution of the spectral observations.

Add the sfhdelayedbq module implementing a delayed SFH with a burst/quench. It is described in detail in Ciesla et al. (2017).

Make sure that the best fit can be computed when the redshift is estimated rather than given as an input.

Add the possibility of computing emission lines through special filters when underlying absorption lines are not subtracted.

Have the nebular module compute and store the intrinsic line luminosities. That way they can be fitted as any flux.

Store the name of the individual lines in the database in order so they can easily be identified and retrieved.

Clarify a bit the description of the redshifting module as we do not always just fit a single galaxy

Completely change the SED cache. Now we rely on the fact that for a given set of modules we only need to store only one model in cache. If there is a cache miss, it means we can discard this one model. In turn the cache key simply becomes the number of modules used, making it easier and faster to access. To check whether the stored module corresponds to the requested one, we store both the model parameters and the SED in a tuple.