qwwad_ef_donor_generic: Add a central-cell correction option

[Alex Valavanis]

Section 5.6 of QWWAD3 describes a central-cell correction to the permittivity, which affects the I4 integral. This could be added reasonably easily as an option to the SchroedingerSolverDonor class (and implemented subclasses).

class SchroedingerSolverDonor
{
public:
void set_screening_length(const double length);

private:
double screening_length;
bool use_central_cell_correction;
}

If the screening length is set, the I4 integral can be computed differently, such that the screening is taken into account. If not, it is computed as normal.