Parallel iMesh data model:

  Machine level: collection of iMesh instances, one per processor,
     communicating through MPI_COMM_WORLD (default) or other specified
     communicator (which is a tag on the mesh instance)
  Processor level: one iMesh instance/processor
  Partition: set of sets with an application-specified tag (by
     name or by tag handle)
  Part: entity set(s) in instance with partition tag specified by
     application

iMesh basic interface extension:
  getEntsByTypeAndTag, getNumEntsByTypeAndTag: get number/entities having
    designated type, tag, and, optionally, tag value
  getSetsByTypeAndTag, getNumSetsByTypeAndTag: get number/entity sets having
    designated type, tag, and, optionally, tag value


Partition level:
  . partition representation: 
    . set which includes part sets
    . marked with "PartitionSet" tag to identify as partition
    . optionally contains "MPICommunicator" tag which identifies
      communicator associated with this partition
  . available as tag conventions: "ProcAssignment" (part-proc,
    entity-proc assignment), "PartitionSet" (identifies handle as a
    partition), "MPICommunicator" (identifies communicator associated
    with partition handle), "ParGlobalId" (for part ids)
  . handled by existing iMesh functions: getTotNumOfParts
    (getNumEntSets), getNumOfPartsArr (getNumEntSets),
    add/rmvPartOnProc (createEntSet/destroyEntSet), create/destroyPart
    (createEntSet/destroyEntSet), 

  . New functionality: 
    . getEntSetsPar (global part list)
    . getDataPar (handles part-proc mapping for non-local parts)
    . getNumEntsPar, getNumEntSetsPar, getNumEntsProcPar,
      getNumEntSetsProcPar
    . getTagsPar, getTagsProcPar, get by tag
    . global reduce on tags
    . get/setGlobalIDSize

Processor level:

  . handled by existing iMesh functions: getNumOfPartsOnProc
    (getNumSetsByTypeAndTag), getPartsOnProc (getSetsByTypeAndTag),
    local ID <-> part handle (getSetByTypeAndTag, getData), what
    partition (
  . entity-part classification: "PartOwner" tag
  . not necessary: 
    . part iterator (don't have a set iterator in iMesh)
    . iMesh instance/handle (partition functions part of iMesh, no
      need for separate iPart interface instance)

Part level:

  . handled by existing iMesh functions: owned entities (getEntities
    on Part handles), getNghbPartIds (getChldn-getPrnts from Part handles),
    part bdry size (getNumChldn on Part handles) 

  . available as tag conventions: getOwnerOfEnt ("PartOwner"),
    getCopies/CopyOfEnt ("RemoteHandle"), getNumOfCopiesOfEnt
    ("NumProcs"), entity categorization ("ProcBdyCategory" =
    in/out/bdy), what partition ("PartitionClassifications"),
    get/set/createGlobalID[Arr] ("ParGlobalId")

  . not necessary:
    . part bdry iterator (don't have set iterators now in iMesh)
    . copies of non-owned interpart bdry entities (entities exist in
      iMesh instance and are not in Part handle)

  . new functionality:
    . addCopyOfEnt, rmvCopyOfEnt (i.e. ghosting functions)
    . disambiguate ownership of shared entity
    . createGlobalID[Arr]
    . getEntHandle[Arr] from global ID

  . convenience functions: 
    . owned/bdy/ghost entities on Part handle