Support getting center of pressure information from the mot-style ground_force_p{x,y,z} columns, so we don't need to create a separate cop file for perturbation analysis.
Support getting center of pressure information from the mot-style ground_force_p{x,y,z} columns, so we don't need to create a separate cop file for perturbation analysis.

git-svn-id: 9e047e1e624cce0a2485e750063112e834f85522@2403 fc29be9c-2cb0-420d-b520-73dbd18add6e

opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core