array function implementation for np.cumprod / np.nancumprod fails with axis

[keewis]

Passing a axis argument to cumprod / nancumprod will make them fail. I tracked it down to

pint/pint/numpy_func.py

Line 62 in 4c3114c

if pre_calc_units.dimensionless:

where pre_calc_units can be a string ("dimensionless"), but a Unit object was expected. The string was created when implement_func was called for cumprod and nancumprod (by the way, cumprod also gets implemented with set_units_ufuncs). We can fix this by passing None as input_units instead, but if feasible I'd prefer "dimensionless" since that is way easier to read and understand. For that, we might need to preprocess the input_units parameter in implement_func.

[hgrecco]

I think is the same as #867 , right?

[keewis]

#867 is about the units of the result if one were to pass axis (and existed before #905), while this issue was introduced in #905 when implement_func was written. cumprod is only defined for dimensionless, so the resulting unit is not a problem.