ENH: extend globalIndex mpiGather to use scalar/label components

- MPI_Gatherv requires contiguous data, but a byte-wise transfer can
  quickly exceed the 'int' limits used for MPI sizes/offsets. Thus
  gather label/scalar components when possible to increase the
  effective size limit.

  For non-contiguous types (or large contiguous data) now also
  reverts to manual handling

ENH: handle contiguous data in GAMGAgglomeration gather values

- delegate to globalIndex::gatherValues static method (new)