Tweak exif data extraction
Tweak exif data extraction

* Use original date time rather than modified date time
* Use Time based assertions when deciding lens, based on purchase dates
(new lens becomes choice over older lens of similar focal length)
* Re-write shutter, strip of `.0` for readability, keep shutters `1/XX`
if they begin with `1`, otherwise use decimals (eg 2/3 is a harder to
read shutter speed)

fofr/paulhayes.photography,fofr/paulhayes.photography,fofr/paulhayes.photography,fofr/paulhayes.photography