Require tune_pme to take -mdrun parameter
The transition away from many binaries in 5.0, and the removal of
symlinks in 5.1 mean there is even less chance that a sensible default
command line for calling mdrun can be provided. In general, mdrun
might be in either precision, with or without MPI, with or without
some custom suffix, inside the wrapper binary or from an mdrun-only
build, or from a custom directory.
Thus, the user is now required to say what command to call to run the
simulation they want to optimize. The MDRUN environment variable
remains as an undocumented and deprecated convenience feature for 5.1.
Fixes #1754
Change-Id: Ia2c278732ef2a79d7304967b230149eca4597888