[py-pysal][py-libpysal] add libpysal and a feature for pysal to pull it in

[couteau]

This PR adds a port for libpysal and a feature for the py-pysal port to pull in libpysal. pysal calls itself a "metapackage", but unlike what you would normally think of as a metapackage, it does not depend on its components, but just creates a package they can be imported through if they are installed. By itself, py-pysal provides no functionality. This PR adds a feature to py-pysal that depends on the new py-libpysal port. libpysal provides the basic functionality of the pysal suite. It can be installed independently or through the lib feature of the py-pysal package, which is a default feature. There are other components of pysal that could also be added as features. They will come with a future pull request.