BUG: vtk write of uniform field in parallel (fixes #2349)

- used low-level MPI gather, but the wrapping routine contains an
  additional safety check for is_contiguous which is not defined for
  various std::pair<..> combination.

  So std::pair<label,vector> (which is actually contiguous, but not
  declared as is_contiguous) would falsely trip the check.

  Avoid by simply gathering unbundled values instead.