easier_terms: update Approximation.describe_geometry()
easier_terms: update Approximation.describe_geometry()

- new return_mapping argument
- make volume mappings local to given region

RexFuzzle/sfepy,RexFuzzle/sfepy,rc/sfepy,rc/sfepy,vlukes/sfepy,BubuLK/sfepy,BubuLK/sfepy,lokik/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,rc/sfepy,vlukes/sfepy,BubuLK/sfepy,lokik/sfepy,sfepy/sfepy,vlukes/sfepy,sfepy/sfepy,lokik/sfepy,lokik/sfepy,sfepy/sfepy