GitLab

Added checks for nonbonded methods
Removed unused functionality from GK force
Moved non-accessor methods from Force classes to ForceImpl classes

An exception is now thrown if the multipole axis type is not Bisector or Z-then-X
Redid velocity initialization in test code

Refactored ForceImpl, lots of KernelImpl subclasses, and other related classes to avoid redundant calculations when requesting a State with both forces and energies