tests for python-interface

To ensure that the python wrapper works as intended we should write a number of unittests in python. As any of these tests also test the library itself it might be worthwhile to replace a larger portion of the unittests with a python variant.

One aspect should be a comparison to numpy-results.

Current errors in the pybind11-bindings:

• xe.Tensor(xe.TTTensor([2])) raises ValueError
• segfault in test_pickle.py with python2-bindings