GROMACS: improve control over PLUMED support (native vs. patching)

[ostueker]

Since GROMACS 2025 one can compile GROMACS with a PLUMED interface without patching it by setting CMake option -DGMX_USE_PLUMED=ON.
Applying PLUMED patches to GROMACS 2025 is still possible in order to get the latest PLUMED functionality.

This PR adds a fourth option 'patch' to the extra-option plumed.

• plumed = None (default) auto detects whether PLUMED is available and do the same as plumed = True
• plumed = True sets -DGMX_USE_PLUMED=ON for GROMACS >= 2025 or try to patch GROMACS with PLUMED's patches for older (<=2024.x) versions (depends whether the loaded version has patches for that version of GROMACS
• plumed = 'patch' will try to patch GROMACS with PLUMED's patches regardless of the GROMACS version
• plumed = False will disable PLUMED functionality, even if it is found (neither apply patches nor set -DGMX_USE_PLUMED=OFF for GROMACS >=2025

I think that's the best of both worlds. It behaves like before for older GROMACS releases (before 2025) and takes the minimal-invasive route (avoiding to patch) when set to None or True.