Error if the unit of CoP is mm in grf.mot

[THEONE-1]

https://github.com/keenon/nimblephysics/blob/bf5c5a174ef236c1a2fe3db9a810bdb87f403d53/dart/biomechanics/DynamicsFitter.cpp#L9656-L9661 Close this issue if I'm wrong but my understanding of DynamicFitter is that it can adapt to .mot files with different CoP units (e.g., mm or m). However, I encountered "Marking trial 0, timestep xxx as probably missing GRF, due to contact heuristic" warning by using mm as CoP unit, probably because the above "padding" is in m.

[nickbianco]

@TheOne-1 I believe COP information is converted to meters before this step, although after checking OpenSimParser::loadGRF, I wasn't able to confirm this.

@keenon, do you know if we convert to meters somewhere else?