add(GmshIO): support for msh2 format on write

The GmshIO plugin now supports writing v2.2 gmsh files when using the *.msh2 extension for the file name.

The GmshIO plugin uses the gmsh c api as a backend to write .msh files from vtkUnstructuredGrids. By default, these files are written in gmsh's 4.1 file format. However, if given a file with a different extension, the behavior of the underlying c api is modified to accomodate that extension. As such, by simply allowing for files with the *.msh2 extension, one enables users to write gmsh files in the 2.2 format.

Default behavior if no extension is provided is still to use the *.msh extension and thus the 4.4 format.