Fix segmentation fault in machine test

Created by: femtobit

The reason for the segfault seems to have been that the Machine instances only store references to the Hilbert instances. Since these were created from Python and stores in the variable hi, they were deleted when hi was assigned another Hilbert instance, in the test code and the machines then contained dangling references.

The return value policy for Machine.get_hilbert is py::return_value_policy::reference, which I think is correct (and this policy should also be specified for Operator.get_hilbert).

The part using Gradient in one of the tests is still commented out because Gradient does not seem to be exported right now (unless I missed something).