Improvements of bhfmm

[HomesGH]

Describe the bug
It is not very trivial to use bhfmm. After looking up the xml syntax for it in the header of this file, I tried to replace the reaction field electrostatic section in a config with the FastMultipoleMethod electrostatic section. Though, this doesn't work (ERROR: Electrostatics section for reaction field setup missing.) and the reaction field section is needed additionally. Inspect this code to see why. I am not really deep into the bhfmm method, but this does not look correct (or at least intuitive) to me.

Things to do:

1. check if bhfmm replaces the reaction field or is an addition
2. Add usage example and doc for bhfmm