API support for Mulliken charges and dipole moment

[agoetz]

Amber users have been requesting to print charges during MD simulations.

To enable printing of charges, the API needs to be extended to pass atomic charge information. Note that currently only Mulliken charges are implemented. Charge and dipole calculation is triggered by adding the DIPOLE keyword.

Changes to the API will require corresponding changes in Amber (sander).

[TrainHeartnetCat]

It would be great to have charge output in API‑mode available. I’ve tried the FBI interface and it supports charge analysis during MD. However, as it was mentioned in the paper, FBI is indeed significantly slower than the API.