Restore array naming behavior for vtkTableFFT
The output columns of the vtkTableFFT filter are now no longer prefixed with
FFT_
and thus are now consistent with the input column names. This previous
wrong behavior was introduced in VTK 9.1.0 and is now fixed.
Also fix vtkTableFFT::SetAverageFFT and improve some variable names
Backport: release
Edited by Timothee Chabat