Add support for ISIS Gtiffs/GDAL labels

[Kelvinrr]

It would be useful for people to run isd_generate on Gtiffs or other GDAL file types with ISIS labels.

I think the simplist way to do this is to update the isis_label class to open using GDAL, similar to how ISIS does it in its code.

[Kelvinrr]

Worth noting, depending on OSGeo/gdal#12916 the label might slightly change. Namely, reading tables. If we exapnd this to ISIS spice mixin.