* DPI is a tuple
* Some EXIF only contains an X resolution for DPI
* Refactor
* Test with no DPI in EXIF
* Handle EXIF with no DPI
* Created with: exiftool "-*resolution*"= photoshop-200dpi.jpg
* Test when not in EXIF, DPI==72,72
* Use X resolution for Y, default to 72,72 dpi
* Created with: exiftool -exif:ResolutionUnit=cm photoshop-200dpi.jpg
* Test for EXIF with dpcm instead of dpi
* Convert dpcm to dpi, and default to inches if unit unknown
