Feature STL support

[timfelle]

This PR adds support for STL files in GEL.

• A STL loader of both ASCII and binary is added.
• A STL writer for both ASCII and binary is added.

Please note the loader will load the triangle soup and stitch them based on the distance 1e-6 * average_edge_length, this should hopefully be relatively robust for merging the triangles. However a message is printed if not all edges are stitched.

Currently this has been tested for:

• a simple box exported from blender, in ascii and binary
• A bunny stl, in binary and converted to ascii by meshlab
• a topopt design i had lying around. (~375 k faces)

Only tested in Ubuntu under WSL.