Workflow:
- define occ geom, without meshing constraints
- sync
- set mesh size (in gmodel)
- resync : we would loose the mesh size spec since occ would tell us that lc = MAX_LC

Ok, so STLs in OCC are still somewhat unstable... Let's default to 0 and enable Geometry.OCCBoundsUseStl when necessary.

This is (unfortunately) much more accurate: even in simple cases
the bboxes of OCC shapes are otherwise not tight

Add tutorial descriptions

See merge request !313