Create proper setup script

[rodluger]

The setup script for multi-planet should use setuptools, as in

https://github.com/VirtualPlanetaryLaboratory/bigplanet/blob/master/setup.py

The current setup script doesn't allow multi-planet to be listed as a dependency of vplanet since setuptools doesn't recognize it as a valid Python package.

[caitlyn-wilhelm]

just pushed to the master branch with the changes!