Local atom sets in compEL / ion swapping code.

Local atom sets in compEL / ion swapping code.

Local atom sets remove depency of domain decomposition on the
computational electrophysiology modules and handle atom index updates
outside of this module.

Additional refactoring, because swap_group has to be a C++ struct now.

Refers to #1972

Change-Id: Ic6ee2a71f5c5c11e161fa26aafbed2764e014daa