Make pysolar numpy-aware

[fsteinmetz]

I understand there may be a will to have pysolar only depend on the standard library. However for some applications it would be extremely useful to use numpy instead of math, in order to vectorize operations, and support array inputs/outputs to most of pysolar's functions. Maybe even support both using something like:

try:
    from numpy import pi, radians, arctan as atan, arcsin as asin # etc
except ModuleNotFoundError:
    from math import pi, radians, atan, asin # etc

[pingswept]

Support of Numpy is desirable, as long as it doesn't become a dependency. The approach outlined above sounds good to me.

Patches welcome!

[pingswept]

Just a note that this is partially completed, as described in #74. I'm not sure what work remains, but help is still wanted.