BUG : fixed transpose error

Now returns the center (row, col)
MNT : over-hauled detector2D_to_1D

 - updated doc-strings
 - might still have transpose errors
 - added support for pixel size
 - corrected kwargs to match core_keys
ENH : added binning helper function

 - function takes an image, a function to convert coordinates to 1D
   and does binning

 - one R2 -> R1 function (radius)