support user-provided function for potential cells in get_ref_coors_general()
support user-provided function for potential cells in get_ref_coors_general()

- new get_cells_fun argument in get_ref_coors_general(), get_ref_coors()

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