Add complex number support to isnan

[kgryte]

This PR

• adds complex number support to isnan by documenting special cases. Namely, if either component is NaN, the result should be True; otherwise, the result should be False. This follows Python, Julia, and NumPy.
• updates the input and output array data types to be any numeric data type, not just real-valued data types.

Notes

• Based on the guidance included in this PR, when applied in conjunction with Add complex number support to isinf #530, a complex number can be both infinite and "NaN". This may not be intuitive for users accustomed to real-valued tests, where a real value is either infinite or NaN, but not both at the same time. Accordingly, users cannot rely on simply using isnan and isinf to identify special results involving complex numbers in a mutually exclusive fashion (something which is commonly done when branching based on IEEE 754 floats).

Reference

>>> z
(inf+nanj)
>>> cmath.isnan(z)
True

>>> import numpy as np
>>> z
(inf+nanj)
>>> np.isnan(np.array([z]))
array([ True])

julia> z
inf + NaN*im

julia> isnan(z)
true

[rgommers]

LGTM. This seems consistent with existing practice, and I don't think there were any remaining concerns with that behavior. In it goes, thanks @kgryte