Add support for an external modal force that sends the motion of (selected) FEM nodes and receives the corresponding loads
Currently, the peer software receives the generalized ("modal") coordinates and returns the generalized ("modal") loads. However, a modal model in MBDyn may need to be coupled with an external solver that needs the motion (displacement, rotation, and their derivatives) of physical nodes. Currently, this can be achieved using the "mapping" option, but for displacements only (mapping of rotations is tricky). The motion of the FEM nodes (including rotations when this applies) can be reconstructed directly by MBDyn and sent as if it were the motion of physical nodes, as a form of physics-based mapping.