Resource path improvements
Make resource path work with everything
- .spd,
- geometries (.ply .obj)
- Textures ...
To do so call the method find_file_from_folders from mrf::io::
For now it works only with MeasuredIsotropicMaterial (altafile) and envmaps.
See commit abdfb1b1f86cfa2204a18ab20811c2d21d233ea9 "Add ext path resources feature"