Added TREx-RGB imagers

[0.19.0] - 2023-08-05

Added

• The TREx-RGB loader, asilib.asi.trex_rgb(), courtesy of C. McKenna.
• asilib.asi.trex_rgb() tests
• Conjunction.lla_footprint tests
• asilib.asi.trex_rgb() to the online documentation.

Fixed

• A bug in Imager.keogram() that calculated the incorrect pixels that is used for slicing images when assembling a keogram. While this bug does not affect ASIs with square pixel resolution, it did for TREx-RGB which is rectangular. The fix slightly modified the keograms, by about a pixel, so I regenerated the baseline keogram plots.
• A bug in Conjunction.lla_footprint where the alt variable was rewritten. As a result, IRBEM did not map to the requested altitude.