Build packages from branches

Created by: Zaharid

We should try to have packages from non-master branches that can be trivially used to e.g. run fits.

At the moment we can upload the packages that get built, but those are indistinguishable from nnpdf-master so that is not very good. Instead we should have:

• Different package names.
• Packages that are mutually incompatible (you can only have one NNPDF package from any branch installed).

Conda used to make that very complicated but now there is a feature that may be just right: https://docs.conda.io/projects/conda/en/latest/user-guide/concepts/packages.html#mutex-metapackages

The idea would be that you can just do conda install nnpdf-flbasis and run that out of the box.