Small bug in precon

I notice the sparse precon matrix P is converted to list-of-lists (lil) format incorrectly:

973        if len(fixed_atoms) != 0: 
974            self.P.tolil() 
975        for i in fixed_atoms: 
976            self.P[i, :] = 0.0 
977            self.P[:, i] = 0.0 
978            self.P[i, i] = 1.0 

P.tolil() does not change a matrix to lil format, but rather builds and returns a new lil_matrix. The line should probably be replaced by self.P = self.P.tolil().

(But actually since self.P is set to csr a few lines further down, consider using a local variable instead of self.P for the processing, as the self namespace is only useful for state that must persist after the method returns, and the lil_matrix is not supposed to persist)

This happens in two places (since that snippet was copied and pasted, which we mustn't do -- please replace any and all duplicate code by programming constructs such as descriptively named helper functions.).