As observed by @R.Boman, katropton was not working with MPI using Gmsh 4.
This was due to the fact that Gmsh 4 does not save the ghost cells by default.
To save those cells, we have to use the option -part_ghosts as discussed in the Gmsh doc.
However, this option is not supported by Gmsh 3 (gmsh -h | grep "ghost" does not find any line).
Therefore, to solve this issue, the code must test the version of Gmsh used and, if the major version is 4 or greater, must add the -part_ghosts argument.
This MR includes the correction of if statements in plat*.geo which prevented the use of newer Gmsh than 3.0.6.
