Instrument Model: Galileo SSI

[tjslezak]

Adds support for the Galileo Solid State Imaging (SSI) experiment imagery.

[tjslezak]

Here is a link to the test image (Chaac Patera, 6413r.img): https://pds-imaging.jpl.nasa.gov/data/galileo/galileo_orbiter/go_0022/i25/io/c052734/6413r.img
I couldn't find where the other test mission data images were referenced, so Travis isn't liking it.
Let me know if you see any errors present. Working on Voyager NAC after Galileo SSI goes through.

[percurnicus]

I have to add the image to planetarytestdata. I've been meaning to add more images anyways so I can get that done today.

Please add the checklist to the top description. I think you have finished everything but it will be easier to know when you fill out the checklist.

[percurnicus]

But thanks! It looks great except for the failing tests which should be fixed once I update planetarytestdata! I'll release new version when Voyager Model is created

[tjslezak]

@pbvarga1 This should work once planetarypy/planetary_test_data#18 is merged.