ENH: add guards for partially populated communicators

- these are primarily when encountering sparse (eg, inter-host)
  communicators. Additional UPstream convenience methods:

  is_rank(comm)
    => True if process corresponds to a rank in the communicators.
       Can be a master rank or a sub-rank.

  is_parallel(comm)
    => True if parallel algorithm or exchange is used on the process.
       same as

           (parRun() && (nProcs(comm) > 1) && is_rank(comm))