add Exception if setIntegrator() is called after an integration step has already been taken
add Exception if setIntegrator() is called after an integration step has already been taken

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