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...
Provide stream that ignores everything written to it

Use this stream when you want a straight code path, but also be able
to disable output at will.
Disable output to console if parameter quiet = true

If the standard output is not desirable, it can be suppressed and
custom output can be written through the timestep_completed event
instead.
Merge pull request #392 from rolk/392_quiet

Add option to suppress output in SimulatorIncompTwophase
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