The nodata points are now always properly filtered out when computing a least square fit. The code has been a little reshaped in order to be closer to the reference document that has also been cited. Some comments have been added. The quadratic fit was wrong and has been fixed.

The statistical tests now cover the least square fits and ad-hoc linear regressions.

The ARC plugin properly builds the file names for reading and writing grids with the "A3D_VIEW" option.