Nonmaxsup fixes/bikeshedding

* Rename (again) orient -> gradientangle, per 2/3 consensus ;-)
* Fix CoordOffset sign to be sign of fraction instead of sign
  of integral part, to avoid zero values on angles
* Fix subpixel parabolic interpolation; was previously moving
  in the wrong direction
* Modify interpolation to also return the minimal adjacent pixel
  value in the direction of the gradient; if this pixel value
  is greater than the value at (x,y), (x,y) is not a local maximum
* Fix comments in interpolation
* If the type of out is a float, return the interpolated value in out
  when subpixel interpolation is requested
Nonmaximal suppression updates

* Added tests
* Fixed angle discretization for row-major images
* Fixed subpixel offset calc
* Changed angle discretization from (0,pi) to (0,2pi).  By testing
  in the direction of the actual gradient first, we reduce the number
  of interpolations in the other direction by ~1/4
* Added thin_edges, thin_edges_subpix.  Right now, these
  just point to thin_edges_nonmaxsup and thin_edges_nonmaxsup_subpix,
  but could be expanded to use other edge thinning methods in the
  future.