Remove restriction on dihdraltypes order

Remove restriction on dihdraltypes order

Wildcards are allowed for dihedraltypes by grompp. But it was assumed,
with only code and no user documentation, that wildcard types come
after fully specified types. If this order was switched, grompp would
silently choose the wildcard matches. Now the match with the most real
atom type matches is chosen.
Added a paragraph to the bonded parameter section in the manual on
dihedral wildcard matching.

Fixes #1901.

Change-Id: I7e44ff19e4d069d1b186ea470438a47d48f1a72d