Spline: fix a few issues with the monotonic() methods

This should fix the common case where the curve is non-constant within
an intervall. I'm not really sure whether it's correct in all corner
cases, though.

this fixes the eWoms test case for the blackoil model which failed in
debug mode due to some asserts incorrectly triggering...
spline: use evalDerivative_() instead of evalDerivative() if the segment index is known

thanks to @bska for catching this!
spline: actually return something in evalDerivative() in the extrapolate case

once more, thanks to @bska to catching this!
Merge pull request #394 from andlaus/fix_spline_monotonic

Spline: fix a few issues with the monotonic() methods