Eradicated admesh from TriangleMesh:

TriangleMesh newly only holds indexed_triangle_set and
TriangleMeshStats. TriangleMeshStats contains an excerpt of stl_stats.
TriangleMeshStats are updated when initializing with indexed_triangle_set.

Admesh triangle mesh fixing is newly only used when loading an STL.
AMF / 3MF / OBJ file formats are already indexed triangle sets, thus
they are no more converted to admesh stl_file format, nor fixed
through admesh repair machinery. When importing AMF / 3MF / OBJ files,
volume is calculated and if negative, all faces are flipped. Also
a bounding box and number of open edges is calculated.

Implemented its_number_of_patches(), its_num_open_edges()
Optimized its_split(), its_is_splittable() using a visitor pattern.

Reworked QHull integration into TriangleMesh:
    1) Face normals were not right.
    2) Indexed triangle set is newly emitted instead of duplicating
       vertices for each face.

Fixed cut_mesh(): Orient the triangulated faces correctly.

tests/sla_print/sla_raycast_tests.cpp

@@ -63,7 +63,6 @@ TEST_CASE("Raycaster with loaded drillholes", "[sla_raycast]")
     sla::DrainHoles holes = { sla::DrainHole{p, normal, radius, hole_length} };
     
     cube.merge(*cube_inside);
-    cube.require_shared_vertices();
     
     sla::IndexedMesh emesh{cube};
     emesh.load_holes(holes);