ENH: improve handling of finiteArea mesh with distributed roots

- in makeFaMesh, the serial fields are now only read on the master
  process and broadcast to the other ranks. The read+distribute is
  almost identical to that used in redistributePar, except that in
  this case entire fields are sent and not a zero-sized subset.

- improved internal faMesh checking for files so that the TryNew
  method works with distributed roots.