Modular simulator: Disable invalid code paths

Modular simulator: Disable invalid code paths

This explicitly disables two invalid code paths for modular simulator:
* The new MTS is not implemented for modular simulator. It is only
  implemented for integrator = md, which by default uses the legacy
  code path. This default can however be overwritten by setting
  the environment variable GMX_USE_MODULAR_SIMULATOR=ON, which would
  result in errors due to the MTS data structures not being properly
  initialized.
* The non-MTTK Parrinello-Rahman barostat is available in modular
  simulator, but not in the legacy simulator. The case that this
  barostat is chosen in combination with other options not compatible
  with modular simulator needs to be caught explicitly.