Port the 2/3D Euler solver from the legacy solvcon code base

[yungyuc]

There are a couple of multi-dimensional Euler solvers implemented in https://github.com/solvcon/solvcon . This issue tracks the progress to port them to modmesh.

The porting will use the branch multidim. Some work of the porting used the multidim branch, but I found it makes tracking related issues harder. I will go back to the master branch.

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• Add placeholder for the 2/3D Euler CESE solver #403 Adds the placeholder for porting code from the old solvcon repository.
• Add 2D unstructured mesh of triangle elements in a rectangle #422 Add a simple 2D mesh file for testing.
• Merge back to the master branch.
• Port code to set up oblique shock wave problem in 2D.