Adding clarifications

[ramirezfranciscof]

These are some parts of the code that I recently found hard to understand and would like to see if there is a clearer way of documenting:

• The subroutine "density" (and therefore, obtain_new_P) does not return the density matrix per se. It returns a matrix that equals the density matrix in the diagonal but is multiplied times 2 in the non diagonal terms.

• The subroutine calc_fock_conmuts does not perform just the commutation, but a base change in the same concerted calculation.

Further stuff to be added.

[fedepedron]

@ramirezfranciscof shouldn't you be dealing with this in SCF?

[ramirezfranciscof]

The first issue has been taken care of in #175 , I didn't look into the second one yet...