Fixes to multicolvar neighbor list

Neighbor list in multicolvar now has three input parameters

TOL - all values less than this are ignored
NL_TOL - all values less than this are ignored until next neighbor list time
NL_STRIDE - frequency with which to update neighbor list

Neighbor list has been tested for DISTANCES and ANGLES.  Still needs more
testing
Transposed MultiColvarFunction functionality from crystallization into plumed

MultiColvarFunction allows you to calculate thing like the average coordination
number for the atoms an sphere centred on a particular atom.  If crystallization
is installed you can also use it to calculate things like the local q4 parameter.
Added functionality to communicator for mpi operations on PLMD::Matrix
Added SPRINT collective variables
Added max vessel