Multicolvars for distance components + register copies of action

This new multicolvar calculated using the XDistances class.  This
class is registered three times for the commands XDISTANCES, YDISTANCES
and ZDISTANCES.  The changes to ActionRegister.h are what allows me
to do this
Change in input to make this regtest run faster

Problem due to very large values for cell vectors with nopbc
Merge branch 'master' of https://github.com/plumed/plumed2