zoom.cxx and origin

[KrisThielemans]

When using even-sized images, there's confusion about how zoom_image works (when specifying zoom factors, as opposed to an image), as it attempts to zoom around the "centre" of the image. This is somewhat described in the FAQ.

this won't be possible anymore once we support non-standard bed positions etc, see #223.

Note: changing this will break backwards compatibility.

[ashgillman]

I think #618 simplifies this a lot. Now we can say that zoom_images merely preserves the location of features with respect to the global physical origin (centre of the first ring), unless an offset is supplied, which acts to translate the image physically.

Regarding "Combining images and projection data" - the gantry_coord system used by ProjData defines (0, 0, 0) as the centre of the gantry, and the physical_coord system used by Densities define (0, 0, 0) as the centre of the first ring (looking into the scanner from the bed). ProjDataInfo is the only place that has the functions responsible for moving between these systems.