Fix numeric locale issue with Legacy VTK readers & writers

Previous implementation is sensible to the numeric locale settings. This can lead to the creation of VTK files with local numeric separators that, for instance, are not compatible with applications like ParaView that sets the "C" locale to avoid such conflicts. This fix set the "C" locale before reading/writing files and restore the previous value when done.

Testing the feature is not easy as it would require that builbots have exotic locale installed. Attached is my testing procedure. main.cxx