inconsistent data types from zmat

[AlexHeide]

The coordinate indices can be lists of sets of indices, lists of lists of indices or 2D numpy arrays of indices depending on the type of zmatrix being constructed. A single datatype should be chosen and the test_zmat be updated.

Currently #15 is using reference values of list[list[str] for zmat and redundant, casts numpy arrays to list for redundant, and uses list[set[str]] for custom zmats.