Correct Type-Checking

[O957]

This issue covers ensuring that numpy and other libraries have their types checked correctly, where applicable. This issue comes from the following statements made by DHM:

• Neither of these is totally robust.
• See https://stackoverflow.com/questions/26921836/correct-way-to-test-for-numpy-dtype
• Suggestion: use issubdtype. https://numpy.org/doc/stable/reference/generated/numpy.issubdtype.html

Regarding:

• if not interval_dates.dtype == np.dtype("datetime64[ns]"): versus
• if interval_dates.dtype != np.dtype("datetime64[ns]"):

See here for the original remarks.