GMX_mpi is not recognised

[SanjiKun-Santhosh]

Hi, Im Santhosh.
I identified a potential problem in the conan.py script.

In the script, at the line 1733, the code checks for the gmx executable but doesn't verify the availability of gmx_mpi. This might cause issues for new users whose system only has gmx_mpi instead of gmx.

Line 1733 ```
program = which("gmx")
if program == '':
print("Gromacs executable was not found")
else:
print("Using gmx executable in %s" % program)