fix dispersion_analysis.py for single requested eigenvalue
fix dispersion_analysis.py for single requested eigenvalue

BubuLK/sfepy,vlukes/sfepy,rc/sfepy,sfepy/sfepy,vlukes/sfepy,sfepy/sfepy,vlukes/sfepy,BubuLK/sfepy,BubuLK/sfepy,rc/sfepy,sfepy/sfepy,rc/sfepy