[xExport] add to GmshMPIIO a file splitting mechanism
A new default parameter is added to xExportGmshMPIIO constructor. This parameter fix the numbers of files to be generated (see comments in sources for precise policy). This addition give a way to split huge results in smaller part more easily handled with GMSH. It might give also extra performance for large number of process by enlarging the number of file treated simultaneously by the file system.
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