Convert test_dimensions.py to pytest
Convert test_dimensions.py to pytest

Seperates all tests out into explicitly parametrised versions.

All public functions in the `dimensions` module are now tested, even
though perhaps `dims_to_tensor_perm` and `deep_map` perhaps don't need
to be public.  There are some tests of the "operator-ket" and
"operator-bra" types now, which didn't exists previously.

There could perhaps be better documentation of what the tensor
permutations are meant to be for the super-operator representations.

qutip/qutip,cgranade/qutip,cgranade/qutip,qutip/qutip