Wfg doc

The documentation on the WFG part, should now be all finished and done. It includes:
- A theorethical background on Zernike modes, Noll's mapping, Zernike Decomposition, Kolmogorov, von Karman, Subharmonics for Kolmogorov/von Karman phase screens
-A small 'user manual' that describes the classes, methods and functions used.