Wrong charges passed from AMBER patch

[GiovanniBussi]

We just discovered that charges passed to plumed by sander (both patch for amber14 and native amber15 implementations) are in not in the expected units. A fix to patch and to native amber is required to divide them by a factor 18.2223 (see here).

Notice that currently these variables only affect DHENERGY and DIPOLE. It should be enough to scale the former by 1/332 and the latter by 1/18.2223 to get a value consistent with other MD programs.

Giovanni