hom: allow passing scalar volume to coefficient classes
hom: allow passing scalar volume to coefficient classes

- new MiniAppBase._get_volume()

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