fix Approximation.get_base() for SurfaceField
fix Approximation.get_base() for SurfaceField

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