OMI_physics_body: Add a center of mass property

[aaronfranke]

I had intended this to be the behavior, but I forgot to explicitly define it as such.

Also change the wording near the start to only mention type and not mass, since we also have linearVelocity, angularVelocity, and inertiaTensor parameters not mentioned, so I don't think we need to mention mass.

[robertlong]

It might be nice to specify centerOfMass as an array to set an explicit offset.

[aaronfranke]

Changed the PR to add a center of mass property instead of defining it as the origin. The default is [0.0, 0.0, 0.0]. There was some discussion about having the default be automatically calculated, but I find that a bit awkward and it's different than the MSFT spec that has the default as [0.0, 0.0, 0.0].

[antpb]

5 in favor, 1 abstain

[antpb]

LGTM

[antpb]