Add different requirement files

[RobertTLange]

Currently there is only one minimal set of requirements in the setup.py file. Each time the user wants to do something "more specialised" they will most likely get a module error stating that they should install an additional package. That. can be frustrating. Instead, I want a set of different requirements.txt as for example in the distrax repository:

requirements.txt
requirements-full.txt
requirements-examples.txt
requirements-tests.txt

Also take a look at their setup.py and make ours more professional.

[RobertTLange]

Addressed in b0ae760.