Draft: Enforce that the HDF5 path is a string

This was a bug I stumbled across. Ideally the keys should not be integers, but it is in principle possible - which causes the file save to fail.

I've seen a few other issues that break this writing. My concern is that it should then fail the following test, which is concerning.

old_result.save(filename="test.hdf5")
new_result = load("test.hdf5")
old_result == new_result

mentioned in merge request !1156 (merged)

Closing this in defference to !1156 (merged)

closed