[Python] make covariance type selectable from the REST interface

[suntzu86]

BLOCKED BY: #378

After #378 makes it so that you can select covariance types through the Python interface, we should make this feature accessible through the REST interface.

1. set up cpp_wrappers/covariance.py python functions for each exposed C++ covariance fcn
2. set up linkers, constants, etc. to make these selectable and buildable through the REST interface
3. check that we do not assume square exponential anywhere in the REST interface, tests, etc.