use NTA_ASSERT

so the check can be turned off for speed by compiler flag,
the checks are only for confinient description, code will not fail
even if empty
use empty() where appropriate

faster than size() > 0 , size() == 0 ,...
unused TRON::norm_inf() removed
Merge branch 'master' into cppcheck-performance
revert 'private', causing error
Merge branch 'master' into cppcheck-performance
Merge pull request #259 from breznak/cppcheck-performance

Cppcheck static code analysis -  performance