Add vtkBandFiltering new filter

This filter decompose a signal into several octave frequency bands. This MR also introduce the new DSP module, standing for "Digital Signal Processing". It aims to gather signal related filters, and especially accoustic related filters. vtkBandFiltering is the only filter in this module for now but it should be more and more populated moving forward.

Make band filtering return 2 values for plotting : it is way clearer to return the same value at 2 different frequencies so it behave nicely when plotting using log scale.