update Variables.setup_lcbc_operators(), .__init__(), FieldVariable.__init__()

- new Variables.has_lcbc_rhs
- rename Variables.op_lcbc, .rhs_lcbc -> .mtx_lcbc, .vec_lcbc
- remove FieldVariable.has_lcbc
set vec_lc to None in LCBCOperators.make_global_operator() when no rhs
update LCBCEvaluator, eval_equations() for renamed operators
add comment to State.get_reduced()
update examples and tests for new LCBC syntax
update meshes/2d/square_quad.mesh to have two cell groups
new examples/diffusion/laplace_coupling_lcbcs.py + test
new examples/linear_elasticity/elastic_shifted_periodic.py + test
update script/gen_gallery.py for new examples

- add custom views for laplace_coupling_lcbcs.py, elastic_shifted_periodic.py
new examples/standalone/interactive/laplace_shifted_periodic.py