allow index in a list in PermeabilityRTerm
allow index in a list in PermeabilityRTerm

- in this way it can be easily modified externally

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