clear() was only required by a unittest to execute two tests directly
after another. This can also be done in a cleaner way by creating a new
CInterpolator instead of requiring a separate method for resetting the interpolator.
This is an intermediate step to have smaller unit tests. It is a trade off
between having many many test executables compared to a few bigger ones. But
this comes a lot closer to what QtTest is meant to be used.
