Improve vtkTableFFT API and behavior

There was some misleading parameters in vtkTableFFT that weren't standard, as well as wrongly scaled result when using the welch method. This MR tries to first address the problem of scaling VTK-wide by adding the proper functions directly in vtkFFT (so upper class can use these tested methods safely), and address the second problem by deprecating useless methods in vtkTableFFT and creating the proper one, based on SciPy API.