modal_analysis.py: new --density option, change material to aluminium
modal_analysis.py: new --density option, change material to aluminium

- increase eigsh() tolerance

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