Add gromacs 2024 ~mpi

Without MPI for two reasons:

• for situations that don't need MPI, threadMPI is often faster
• to prepare for eventually adding with +plumed and +cufftmp. Even though the +plumed appears in the name, it's less confusion (since there will only be one -mpi* module.

stacks/pinot-noir/definitions_stack.yaml

@@ -99,7 +99,7 @@ definitions:
 
   - gcc_blas_codes:
       - armadillo +hdf5 ^arpack-ng ~~mpi ^hdf5 ~~mpi
-      - gromacs ~mpi ^fftw ~mpi
+      - gromacs ~~mpi ^fftw
       - octave
 
   # -------------------------------------------------------------------------