Resolves #231 by always using center of mass as the meaning of vessel position.
The root cause of the problem was an inconsistency in how different places in
the code defined "the" position of a vessel.  It was sometimes using the
vessel's root part and sometimes using the vessel's center of mass.  Now it
uses the vessel's center of mass for everything it does.
Merge remote-tracking branch 'Dunbaratu/positions-all-use-CoM' into develop