Graph coloring using scotch

[Algiane]

Implementation:

• Add possibility to call scotch instead of Metis for initial partitioning (nodes of graph are mesh elts)
• Add possibility to call scotch instead of Metis for interface migration (nodes of graphs are groups)
• Add weighted graph support
• Add Scotch and PT-Scotch partitionning

Checks:

• Robustness to non contiguous partitions (if PT-Scotch is used)

If Scotch graph is stored under the same format as Metis graph we only have to rename the PMMG_graph_meshElts2metis, PMMG_graph_parmeshGrps2metis and PMMG_graph_parmeshGrps2parmetis functions.

Otherwise we will have to create new fonctions to store the graph at suitable format and to refactorize common parts of the construction of graphs at scotch and metis format.