unify diffusion velocity evaluation terms
unify diffusion velocity evaluation terms

- de_diffusion_velocity, di_diffusion_integrate -> ev_diffusion_velocity

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