ENH: direct ensight output of float/double

- since ensight format is always float and also always written
  component-wise, perform the double -> float narrowing when
  extracting the components.  This reduces the amount of data
  transferred between processors.

ENH: avoid vtk/ensight parallel communication of empty messages

- since ensight writes by element type (eg, tet, hex, polyhedral) the
  individual written field sections will tend to be relatively sparse.
  Skip zero-size messages, which should help reduce some of the
  synchronization bottlenecks.

ENH: use 'data chunking' when writing ensight files in parallel

- since ensight fields are written on a per-element basis, the
  corresponding segment can become rather sparsely distributed. With
  'data chunking', we attempt to get as many send/recv messages in
  before flushing the buffer for writing. This should make the
  sequential send/recv less affected by the IO time.

ENH: allow use of an external buffer when writing ensight components

STYLE: remove last vestiges of autoPtr<ensightFile> for output routines