Debugs of the script for mesh format conversion Gmsh -> Specfem2d

[mnagaso]

This is an important update for EXAMPLES/Gmsh_example_CPML_MPI.

The python script for converting the mesh file format from Gmsh to Specfem2D had some bugs on:

• writing node ids.
• dupilcated elements' edges definitions between acoustic-elastic boundary (thus no energy transition at the interface).

Additionally, a new function has been added for inserting a transition layer between PML layers and a non-PML domain, which may useful to stabilize the simulation.

All the debugs are on the scripts in this example, thus no effect on the main code.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 54.63%. Comparing base (32c7839) to head (9739716).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #1211 +/- ## ========================================== - Coverage 54.64% 54.63% -0.01% ========================================== Files 164 164 Lines 30233 30233 ========================================== - Hits 16520 16519 -1 - Misses 13713 13714 +1 ```

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