Update MeshIO for GeometryBasics refactor

[ffreyer]

(I moved the branch to MeshIO for this one)

Changes for https://github.com/JuliaGeometry/GeometryBasics.jl/pull/219

TODO:

• [x] 2dm
• [x] gts
• [x] ifs
• [x] msh
• [x] obj
• [x] off
• [x] ply
• [x] stl
• [x] test (locally)

Closes #94 (materials, face groups/objects now loaded with obj files) Fixes #80

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 72.24880% with 58 lines in your changes missing coverage. Please review.

Project coverage is 74.72%. Comparing base (9f452ff) to head (8666d91). Report is 28 commits behind head on master.

Files with missing lines	Patch %	Lines
src/io/obj.jl	76.28%	46 Missing :warning:
src/io/ifs.jl	0.00%	6 Missing :warning:
src/io/gts.jl	0.00%	5 Missing :warning:
src/io/ply.jl	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #97 +/- ## ========================================== - Coverage 77.68% 74.72% -2.97% ========================================== Files 11 10 -1 Lines 623 736 +113 ========================================== + Hits 484 550 +66 - Misses 139 186 +47 ```

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[ffreyer]

Added the Sponza model from https://casual-effects.com/data/ with only two of the groups/sub-meshes present and all textures removed. That should be enough to test correct loading of group names, "usemtl" tags,mesh.views and some mtl file parsing, without excessive bulk.

[SimonDanisch]

This is a breaking change, that changes the load method:

[ffreyer]

Oh, true. I didn't have that type at some stages of doing the refactor, so I probably change it then. But I think this is also more in line with other load methods

[ffreyer]

obj, ply, stl, msh, off and 2DM use this already