File with support functions and methods

These function, used to be part of the zernike.py file, split for
readability. Moreover, others may benefit from it. Added a circular
aperture function and a function to create a square spatial grid.
Added a ZernikeWave class

This class allows, to add/remove/adjust modes to your liking. When you
are all done, hit createWavefront(...) and the final result is ready.
For ease, you can quickly plot the result of the modes or the combined
wavefront from the class.  Finally, the class also allows for
decomposition a wavefront in zernike modes, with
decomposeWavefront(...).
Updated test WFG features

New additions: test the zernikeWave class. Wavefront generation as well
as wavefront decomposition.