ENH: use mapDistribute linear construct order in a few places

- simplifies code by avoiding code duplication:
  * parLagrangianDistributor
  * meshToMesh (processorLOD and AABBTree methods)

BUG: inconsistent mapping when using processorLOD boxes (fixes #2932)

- internally the processorLODs createMap() method used a 'localFirst'
  layout whereas a 'linear' order is what is actually expected for the
  meshToMesh mapping. This will cause of incorrect behaviour
  if using processorLOD instead of AABBTree.
  A dormant bug since processorLOD is not currently selectable.