Remove extra constant grave field application

PHYS-16 #time 5m
Undo previous change. That was stupid.

PHYS-16 #time 5m
Add new more complete calcForceGrav method

PHYS-16 #time 20m
Using calcForcegravNew method in main loop

PHYS-16 #time 10m
Add CoM method to MyShape.

PHYS-16 #time 25m

This allows the same getVectorToObject method to work for both
typical MyShape objects and Quadrants.
Eliminate vecToQuad and use general method instead

PHYS-16 #time 15m

This simplifies a lot of Interactions code.
Delete pointless CoM call

PHYS-16 #time 5m
Add getDistanceToObject method

PHYS-16 #time 10m
Remove old calcForceGrav method

PHYS-16 #time 10m
Rename calcForceGravNew to calcForceGrav

PHYS-16 #time 5m

This was always the intended name, I just had to slowly back out the old
version of this method first.