Add preserveNumberingMsh2 as a command line option

This branch includes an additional command line option named -preserveNumberingMsh2 which is complementary to the -save and -part_split option. Like the similar advanced option available in the GUI, this option, it allows to preserve element numbering in MSH2 format between a partition mesh written in both the one-file format and the split format. Note however that it will break meshes with multiple physical groups for a single elementary entity.