The VTK legacy writers can now write out the older
4.2 file format (as well as version 5.1). Use the
method SetFileVersion() to control this. The VTK
legacy readers have also been updated to report
the FileVersion of the file they read. Major and
minor version numbers are also provided.

The major difference between file versions 4.2 and 5.1
is the format for cell arrays (which affects vtkPolyData
and vtkUnstructuredGrid and related classes). In version
4.2, cells were listed using a count of vertices, followed
by the vertices, repeated for each cell. In version 5.1,
in order to support 32-/64-bit cell arrays, and provide
random access, the cell array is produced in two parts:
1) the list of vertices for each cell, and 2) for each
cell, an offset array into the vertices list which
indicates the starting location for each cell's vertices.

Expanded test to unstructured grids