update IGField.get_econn() for surface connectivities
simplify IGField.get_dofs_in_region_group() by using .get_econn()
new get_facet_axes()
new solve() convenience wrapper
new create_boundary_qp()
new get_surface_degrees()
update IGField.set_dofs() for DOFs given by function, update docstring

- initial implementation
update examples/linear_elasticity/linear_elastic_iga.py for EBCs by function
update custom view for linear_elastic_iga.py example in script/gen_gallery.py
Merge branch 'iga-ebc'
script/gen_iga_patch.py: plot 1D basis along patch mid-line