Add feature to automatically select the alchemical ion with largest distance

[evohringer]

This feature becomes more important when using the SAMS protocol, because the obtained free energy becomes dependent on the position of the alchemical ion for host-guest systems.

In general, it is a very minor change and should not affect the rest of the package.

Further details can be find in this ChemRXiv preprint and will be available soon in JCIM.