Gaussian nuclear model with fractional charges

[arturosme]

Possibility of using fractional charges with gaussian nuclear model at the same time (new NUC_MOD_OF). To choose between the different cases, a bitmask has been implemented using the last bit of atm(NUC_MOD_OF, nuc_id)-1 from a given atom.

• Use zeta: for modes GAUSSIAN_NUC and FRAC_GAUSS_NUC.
• Use fractional charge: for modes FRAC_CHARGE_NUC and FRAC_GAUSS_NUC.