Nfw

Bump version number due to pypi filename problems. No changes in functionality between 0.3.0 and 0.3.1.

Moved to Github actions for testing. Much wider test coverage on platforms other than Linux. Deprecated Python 2.x (although it may stil work).

This is the initial release of the python NFW code to compute properties of the NFW halo and convert quantities between different mass definitions and mass-concentration relations. Although the code is well tested, this release is tagged v0.1 because I am contemplating various API changes for future versions.