Updated mesh reading for Gmsh version 4

[jploveless]

celeri_ui was written for Gmsh version 2 files. We're no longer using these, and the modern version 4 files are formatted really differently. I hacked together some changes that might not be perfect, but meshes are now displaying, at least when loaded one-by-one.

The previous version of Gmsh files made things easier, specifying the dimension of mesh elements as the first number of each line within the $Elements block. The new version is meant for more of a line-by-line parsing, with some lines specifying the dimensions and then the number of elements n of that physical section, with the next n lines just containing the vertex indices.

To accommodate this knowing no TypeScript, I find the $Elements section, then find the index where the 2-D elements (triangles) start, then filter out any lines that start with a 2, which indicate subsequent blocks of 2-D elements (but not their actual vertices). This seems reasonable, because Gmsh always (?) defines 0-D and 1-D elements in a .msh file, and so no triangle will have the index of 2, and therefore we won't be missing it when parsing the element indices.

This is not a great way to read a mesh, but really all we need to do is display it, so I think it's fine. I also had to include a wrap-to-360 statement when reading node longitudes.

[jploveless]

Oh, this definitely breaks support for old versions of Gmsh, and so the GitHub action tests are failing.

[jploveless]

Edited the toy coordinates and nodes in mocha/tests/FileOpenerTests.spec.ts and now tests are passing.