Nonmaximal suppression fixes

* Remove extraneous "-1" from interpolation function so that
  we interpolate the correct value; also adjust pad
* Modify algorithm so that it checks the adjacent pixel value
  in addition to the interpolated pixel value.  This removes
  the need for the morphological thinning at the end.
* Remove comment about morphological filtering
* Fix explanation to describe what the functions actually do
* Reverse subpixel values so that pt.x and pt.y always refer to
  x and y axes, respectively