Create unittests

We need unittests for STELLA. This could be done in C++ (using boost::test or the google test suite), or it could be delayed until after we have initial python bindings.

My feeling is that we should wait and do it in python; see https://docs.python.org/2/library/unittest.html. There are examples in e.g. https://github.com/LSST/afw/tree/master/tests