Expand raster filetype support

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• Add gpkg (Geopackage Support #11), also jp2, tiff, nitf to the whitelist
• Map alternate extensions through a dict: {"tif": "tiff", "ntf": "nitf"}
• Ensure tests exercise each of the formats in the whitelist

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This was addressed by #39 which was merged not in this repo, but at GeoNode#47

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We don't map through a dict right now and jp2 is unsupported in importer's test environment and is not a default driver in GDAL, unlike nitf, but there is nothing to prevent it from working with deployments that put it in THEIR whitelist; the actual intent was addressed here