SimplePCI TIFF: evaluate metadata mappings

[melissalinkert]

@mabruce notes that curated/simplepci/qa-17305 has an exposure time visible in original metadata, which is not mapped to Plane.ExposureTime.

Several of the INI keys in this file are different from what SimplePCITiffReader typically expects. QA 17305 was submitted for #2537, so it's known that this file was a little different from older SimplePCI TIFFs. Particularly since this is an older reader, it's likely worth re-evaluating our sample data more widely to check for any other places where metadata is incorrectly or incompletely mapped.