Modified the way groups are read in in multicolvar

You can now read in groups with names other than GROUPA, GROUPB and
GROUPC.  Also output of the atoms in groups is neater.  Lastly I realized that I
had accidentally added some crap to regtest/multicovar/rt45 - I have
now deleted this.
Added BRIDGE colvar, which is equivalent to PLUMED1 waterbridge