Grids for the test-suite

[KnutAM]

Much of Ferrite's code is only tested for grids from the generate_grid function, but @koehlerson and I discussed that it would be valuable to have some grids available for testing that unstructured, stemming from e.g. Gmsh and Abaqus.

We recently found some code, not yet in Ferrite, that worked well on grids from generate_grid, but not for unstructured grids.

One suggestion would be to have a list of different types of relatively small grids for use with testing, both with the same cell type and mixed celltypes. One possibility could be to define something like

const test_grids = Dict(
 "GenTriangle" => generate_grid(Triangle, (3,3)),
 "GmshTriangle" => togrid(joinpath(@__DIR__, "triangle.msh")),
 "AbaTriangle" => get_ferrite_grid(joinpath(@__DIR__, "triangle.inp")),
 "GmshTriQuad" => ...
 "AbaTriQuad" => ...
)

just to make sure to cover some special cases during testing.
This would also serve as "reverse-ci" (cf. #797) for the mesh parsing.

Thoughts?

[termi-official]

Maybe we can just dump some problematic unstructured grids in a test assets folder or hand-generate them tailored for specific corner cases to cover? Otherwise we might need to start thinking about randomization of the tests, if we want to test all combinations of every type of mesh parser+every type of element combination+every type of unstructuredness.

[termi-official]

First attempt in #820