Martin's monster merge part 3/N: rework tests to avoid npz files

This removes the .npz files and their generating code from the repository and simply generates the test data on the fly.

NOTE: In test_gl_space.py, I had to mark the weight tests as "expected failures", since the new tests already check for the correct weights, whereas Nifty still computes incorrect weights due to the bugs in libsharp_wrapper. This will be fixed in subsequent merge requests.

To be reviewed/applied after parts 1 and 2.