add develop and clean make targets

[samuelallan72]

• clean to remove everything from a make develop
• develop to install inplace (same as python setup.py develop

[samuelallan72]

There seems to be multiple ways of installing/running various things now: the Makefile, python setup.py <command>, pip <command>... We have more make targets now, but they aren't documented in the readme development instructions. Not sure what to do about it.

[samuelallan72]

Also should be noted that the make commands all assume a virtualenv has already been loaded. I think this is good because we don't want to assume the location/python version of a venv, but this isn't documented yet either.

[godwhoa]

This'll do