ENH: add some convenience methods in ensightFile, ensightGeoFile

- Some commonly used write methods that are independent of
  the calling context (ie, 2D/3D data, geometry, fields)

- Provide singleton null() for ensightFile, ensightGeoFile.
  Can be used for MPI slaves that need a file reference for their
  methods, but will never write to it, but it is also reasonable
  to use an autoPtr with rawRef() for that as well.