values are not monotonically increasing. Closes bug #2831634.
- Did some related code cleanup and fixed code duplication between
Filter::sortedCurveData() and Interpolation::sortedCurveData().
- Removed lots of unused files, which were either obsolete, redundant copies in
src/future or future code not yet being used. This should make it somewhat
easier for people to grasp the current code base.