Fix partial_tempering with new gromacs

[GiovanniBussi]

As I realized now reading GROMACS manual, GROMACS changed the way wildcards in dihedrals are processed.

• Up to 5.1.3, the rule was to accept the first match
• In more recent versions, the rule is to accept the match with the lowest number of wildcards.

The partial_tempering.sh script should be fixed to implement the new rule. By the way, the new rule makes also more sense to me.

Notice that this problem should be easy to detect by comparing energies obtained with scaling factor 1.0 and energies obtained with the original topology, and nobody reported it, so it might be not important in practical cases.