Unify external potential contributions

[fsmoncadaa]

Right now we can compute three different type of "external" potentials: linear electric field (for polarizability), harmonic (for QDO), and linear combination of GTFs (for arbitrary potentials like He-C60). In the code, we have a variable: CONTROL_instance%IS_THERE_EXTERNAL_POTENTIAL, that in the SCF module is related only to the linear combination of GTFs. I want this variable to handle the three different contributions (and maybe add a flag for each specific type)