Handle GOES-16 Full Disc images

[lesserwhirls]

GOES-16 full disk images use the Geostationary projection and claim to follow CF. However, the CF specification states that the mapping coordinates must be in radians, whereas GOES-16 stores them in microradians.

I've started updating the projection and transform to handle this not-so-cf-y-ness and is a work in progress. This work is being done on 4.6 and should transfer to 5.0 cleanly.

Example files: any of these will do.

[ethanrd]

CF standard names have canonical units which means any units may be used as long as the units used are convertible to the canonical units.

If I'm interpreting the CF Geostationary projection section correctly, it is redefining the units for variables with standard names projection_x_coordinate and projection_y_coordinate from meters (m) to radians. Unfortunately, it says "their units are radians" rather than "their canonical units are radians".

I think that text should probably be updated to specify "canonical units" not just "units". I will email the CF list to see what others there think.

[lesserwhirls]

Cool, thanks @ethanrd! BTW - the changes on my branch allow the GOES-16 data to be geo-referenced properly in ToolsUI. Just need to add some tests.

[lesserwhirls]

Fixed in #1023