Add photon

Created by: niclaurenti

• Do a fit with Theory 133 (3.1)
• Compare with a fit with Theory 53 (3.1)
• Add tests (and fix tests) to this PR
• add fiatlux to conda recipe
• add eko to conda recipe
• restore top-level imports
• add documentation
• (optional) make fiatlux accept dict as input
• Run black and isort using pyproject.toml from master