Renamed detector2D_to_1D to img_to_relative_xyi
-----------------------------------------------

The previous function name was rather obtuse as it did not adequately
describe the function

Change API of img_to_relative_xyi
---------------------------------

from ::

  def img_to_relative_xyi(img, calibrated_center, pixel_size=None, **kwargs)

to::

  def img_to_relative_xyi(img, cx, cy, pixel_size_x=None, pixel_size_y=None):