docs: add missing figure
replace alphas8bit with alphas

- word_free with alphas8bit as default argument value in
  get_standard_type_defs() and list_dict() breaks sphinx docs generation on
  platforms with non utf-8 locale
use os.sep instead of '/' for the pathname separator in import_file()

- fix portability issue
fix plot_nurbs_basis_1d to use ax argument
clean up whitespace
docs: new section on exploring the code in developer guide
new Region.kind_tdim attribute, new .set_kind_tdim(), update .set_kind()
make ig argument optional in Region.get_entities(), signature change
update Region.contains() to use other region kind entities
make sure entities are initialized in Region.get_entities()
fix extend_cell_data() for several groups, new average_surface argument
update Region.setup_from_highest() - new allow_lower argument

- works also when the highest dimension of entities is lower than requested
- fixes selectors like 'vertices of surface *f vertices in (x < 0.0)'
new Region.finalize(), update Domain.create_region()
fix region definitions in tests and examples as reported by Region.finalize()
change keys in transform_epbcs()
report unknown LCBC kind in LCBCOperators.add_from_bc()
new general term