change parameter names to beta, gamma in NewmarkTS, update docstring
change parameter names to beta, gamma in NewmarkTS, update docstring

- fix use of beta (former beta2)

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