remove unused meshes in meshes/quantum/
update meshes/quantum/square.geo
new meshes/quantum/square.mesh (corresponds to meshes/quantum/square.geo)
update examples/quantum/ - use meshes/quantum/square.mesh

- update quantum_common.py to use square.mesh, add docstring, increase
  approximation order to 2, update common() arguments
- new arguments to define() in examples: n_eigs, tau
update schroedinger.py: remove mesh generation options, simplify

- remove --create-mesh, --2d, --mesh, --mesh-dir options
- new --n-eigs, --tau options for updated examples/quantum/
- update docstring
update test_install.py for meshes/quantum/ and schroedinger.py changes
script/extract_surface.py: fix default value of --mesh option