WFS update, and added function for rectangular setups in the main

Increased the readability of mainWFS.py.
Normalized the intensity distribution and the lensletCentx/y.
Added support length for FFT in the parameters for the sensors.
Added function in main to handle rectangular lenslet configurations.
 
I don't think this function belongs to the main. The main could (most likely shouldn't) import it from wfs, but it shouldn't be defined in the main.
It would be nice if this function also returns only the `lensCent` vectors (or matrices), and not the whole sensor parameters. In that way it can be unit-tested on its actual functionality (the generation of the arrays).