Adds new img_extent attribute to ginifile dataset

[kgoebber]

This pull request adds an attribute 'img_extent' when using the to_dataset() function when using a gini satellite image file.

I was working on creating a satellite image and it seemed that a nice attribute to have for any gini file would be the extent of the image in image coordinates. This alleviates the need of the user to bring in the x and y coords simply to use it in the extent keyword within imshow. This also allows easier use to use Cartopy coordinate transformation with set_extent by being able to use lat/lon values and convert to projection coordinates.

I also added a test to test_gini.py for this new attribute.

[jrleeman]

I'm a fan - that was one clunky thing dealing with satellite data and this is a nice solution!

[dopplershift]

👍 for the functionality. The only thing I'm trying to see right now is if there's a way to encode this information using the CF-conventions

[dopplershift]

(oh and 💯 points for adding it to the test)

[dopplershift]

Just a thought: would having an actual_range attribute on the x and y coordinate variables get you there? That seems to be the only way within CF.

Just floating the idea. We're going to need to tackle this problem in a different manner anyways for the future, as GOES-16 data from NOAAPORT don't have any such bounds information.

[jrleeman]

I guess you wouldn't have to pull out x and y, but it wouldn't be as slick of an interface.

[dopplershift]

Agreed. I guess my point is that we can solve this for GINI files (and I'm happy to merge this as is just to make that easier), but the bigger bang for our buck is to make it work generally for a lot of different data--like GOES-16. Just seeing if there's something more standard that gives us comparable benefit.

[kgoebber]

I say let's go with this solution for now...what format(s) are critical here to find the commonalities (AREA, GINI, Raw)?

[dopplershift]

And one more: GOES-16 netCDF4.