ENH: globalIndex gather ops with reduced communication (#2332)

- for contiguous data, added mpiGatherOp() to complement the
  gatherOp() static method

- the gather ops (static methods) populate the globalIndex on the
  master only (not needed on other procs) for reduced communication

- rename inplace gather methods to include 'inplace' in their name.
  Regular gather methods return the gathered data directly, which
  allows the following:

      const scalarField mergedWeights(globalFaces().gather(wghtSum));

  vs.
      scalarField mergedWeights;
      globalFaces().gather(wghtSum, mergedWeights());

  or even:

      scalarField mergedWeights;
      List<scalarField> allWeights(Pstream::nProcs());
      allWeights[Pstream::myProcNo()] = wghtSum;
      Pstream::gatherList(allWeights);
      if (Pstream::master())
      {
          mergedWeights =
              ListListOps::combine<scalarField>
              (
                  allWeights, accessOp<scalarField>()
              );
       }

- add parRun guards on various globalIndex gather methods
  (simple copies or no-ops in serial) to simplify the effort for callers.