Misc fixes 10 numdiff

add a minimum level to epsilon in numerical derivatives, see issue #972
adjustment to test that sometimes fails

typo in commit message


there were several times test failures on TravisCI, The precision in some calculations is a bit lower with python 2.7 and numpy 1.6.1 (TravisCI) or numpy 1.7.1 (my tox environment)
precision for cs hessian is still 2.5e-10