calculate velocity of center of mass alongside its position
merge PlanetarySystem.centerOfMass into PlanetarySystem.center
add subtractCenterOfMass function; subtract center of mass on calls to PlanetarySystem.reset
fix bugs in subtractCenterOfMass
Update COM continuously in edit mode.
Add lerpv for lerping vectors.
Add lerp flag to calcCenterOfMass.
CalculateCOM then subtractCOM from bodies on tick while editing.
Move edit functions call to planetarySystem.reset out of timeout (otherwise subtractCOM wrecks bodies initial positions when edit is clicked).
Merge branch 'dev'
Merge branch 'master' into gh-pages