Hugin should write projection info to output

When Hugin creates an output image it discards all the photometric and geometric information it knows about this image. Hugin ought to write metadata to the image that would allow Hugin to use the image as input at some time in the future.

Important information is:

 1. Horizontal field of view.
 2. Projection and any relevant projection parameters.
 3. Cropping information, since otherwise the hfov isn't usable.
 4. EV.
 5. EMoR parameters since these are effectively transferred from the first photo in the project to the output.

We could create fake sensor and focal length info, but this would only be credible for rectilinear output. So probably the only way to do this would be to write to the EXIF notes section much like qtpfsgui/luminance, e.g. this is typical qtpfsgui output (this is the actual format):

 Qtpfsgui 1.9.3 tonemapping parameters:
 Operator: Mantiuk
 Parameters:
 Contrast Mapping factor: 0.1
 Saturation Factor: 2
 Detail Factor: 1
 ------
 PreGamma: 0.636

We already require exiftool, this info can be added to the existing exiftool command in the .pto.mk stitching Makefile