[feature] Reduce 3d input file size

[dmonah]

Is your feature request related to a problem? Please describe.
I want to run a number different experiments all with the same 3d mesh. My mesh is quite large and so each of my input files (which contains lists of triangle vertices and indices) takes up a relatively large amount of disc space.

Proposed solution
Rather than list the same vertices and indices in every input file, it would be useful to have a single mesh file containing these values and just require a file path in the input file.

[dmonah]

I can work around this myself, I am just recording the suggestion here.

[drobnyjt]

This is a good idea; I'll think about how to best implement it. It'll require some work to not break backwards compatibility, but that should be possible.

[dmonah]

I implemented a quick hack (sorry, still can't push any code) where I simply added an additional TOML variable called "mesh_file_path" to the geometry input expected by the parry3d geometry class. The default value for this field is an empty string. Now if the value is an empty string, the continues to read vertices and indices as normal (so old files continue to work as expected with no modification required). If a mesh file path is provided, the code reads vertices and indices from here and simply ignores the vertices and indices values in the input file (so new files can leave these blank).