Rename projection operators to distributors

See merge request ift/NIFTy!224

# Conflicts:
#   demos/wiener_filter_via_hamiltonian.py

for some reason Nifty is built with python2 but better_apidoc.py is executed with python 3 in the CI; therefore it does not find the nifty library to generate it's documentation; so now testing run with python2 on both sides

Merge Mihai's documentation enhancements

See merge request ift/NIFTy!223