fix plot_dofs option in save_basis_on_mesh() for no element groups
fix plot_dofs option in save_basis_on_mesh() for no element groups

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