openfe plan: make partial charges output clearer

problem

Currently, if your molecules already have partial charges assigned, you will still see the following output:

Planning RBFE-Campaign:
assigning ligand partial charges -- this may be slow
Generating charges: 100%|██████████████████████| 10/10 [00:00<00:00, 331.31it/s]
	Done

This could be confusing and lead a user to think that partial charges are being reassigned.

expected behavior

If partial charges are present, output "partial charges present, will not assign charges" or something similar.