Gmsh example updated for the latest Gmsh and Specfem2D

[codecov[bot]]

Codecov Report

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

Comparison is base (c7133e5) 54.64% compared to head (c759143) 54.64%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #1204 +/- ## ======================================= Coverage 54.64% 54.64% ======================================= Files 164 164 Lines 30233 30233 ======================================= Hits 16520 16520 Misses 13713 13713 ```

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[danielpeter]

great example, many thanks!

two suggestions for future use:

1. let us choose a different name, to make the name a bit more explanatory about the example setup.
   -> i can do that as i'm going to create some new subfolder structure in the EXAMPLES/ similar to the 3D_Cartesian version, where we have benchmark, applications and reproducible_study subfolders.

2. the Par_file ordering has changed slightly in the meantime, the nbmodels section is now mostly only used for internal meshes, whereas external meshes should use a MESH/nummaterial_velocity_file instead to define the material properities. you will find such an updated usage in the EXAMPLES/Gmsh_example_Stacey_MPI. this is to make the look-and-feel of the examples using external meshes in 2D and 3D_Cartesian versions more similar.
   -> i'll do that when updating the EXAMPLES/ folder soon...

[mnagaso]

Hi @danielpeter,

Thank you very much for merging my PR and explanation on how the material parameters should be set with external mesh currently! I will modify my example to replace nbmodels to use MESH/nummaterial_velocity_file then reopen a merge request soon.

Thanks!