Merge #940

940: Reimplement cumulative product NumPy functions r=hgrecco a=jthielen

This PR resolves two issues from #905 that were identified in #939:

First, `convert_arg` in `numpy_func.py` presumes `pre_calc_units` is a `Unit` or `None`, but there are instances where it was passed a string by way of `inputs_units` in `implement_func`. Now, any `inputs_units` as a string are converted to a `Unit` before being passed to `convert_to_consistent_units`.

Second, the cumulative product functions were using `implement_func` with `input_units` set to something other than `None`, which was incorrect, since these functions have arguments that should not have units (namely `axis`). These functions have been reimplemented accordingly.

- [x] Closes #939 
- [x] Executed ``black -t py36 . && isort -rc . && flake8`` with no errors
- [x] The change is fully covered by automated unit tests
- ~~Documented in docs/ as appropriate~~
- ~~Added an entry to the CHANGES file~~ (fixup to previous change: #905)


Co-authored-by: Jon Thielen <github@jont.cc>

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