m-resolved RPA implemented
Better parameter values in denssim.py
Typo fixed
Ecut and Mmax added to graphene constructor param list
Reference to self.F in density.py removed
Changed normalisation of the initial chi value

The old norm (chi_u**2 + chi_d**2 == 1) was giving poor results for very
large angular momenta: chi_u and chi_d might become so small that their
squares are zeros.