* Added Color Balance image filter
* Some changes in the interpolation classes:
  - changed the names to reflect that the classes work on 1D data
  - moved some functionality from the NearestNeighbor-, Linear- and Cubic-SplineInterpolator1D classes to the base class BaseSplineInterpolator1D
* Fixed a bug in the HSL Color Replacement and the HSL Keying image filters (added code to initialize the cubic spline interpolator to make it periodic)
* Moved some functionality from the levels image filter to the imgproc library (the code that generates a lut from the given gamma, input black and white points, and output black and white points)