Remove a force buffer argument from IForceProvider

Remove a force buffer argument from IForceProvider

For the foreseeable future force providers will produce forces that
should not go into the coord x force type virial calculation.
Therefore the old force argument is now removed and the old f_novirsum
argument is renamed to force. Also removed the confusingly named
withoutVirialContribution_ container (because providers should actually
provide the virial contributions).

Change-Id: I580ac2cef31da6c191a0852c1f3be0c9e09ea1b0