dispersion_analysis.py: support computing all eigenvalues
dispersion_analysis.py: support computing all eigenvalues

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