Separation of core and optional dependencies

[a-r-j]

I'd recommend splitting dependencies into groups like this

This makes it easier to keep track of what is need for testing, dev, docs, prod etc.

Relevant lines for setup.py

[a-r-j]

N.B. these dependencies should be added in the appropriate place:

pip install --no-deps rxnfp
pip install --no-deps drfp
pip install transformers
pip install gpflow grakel