Changed in GaussLaguerre function

[GubioGL]

#71 LG doughnut mode. I detail better in issues #71 the need for this change

I changed only that part, not calculating the R and PHI with the function, because in the part of the phase of the .mgrid_polar function there is a "+ _np.pi", which shouldn't be here.

In order not to change that function, it is simpler to just change it here. I changed from cos( ..) to exponential(..).