Removed the RGB color normalization in the loader

[0.20.6] - 2024-02-18

Changed

• Removed the RGB normalization in the trex_rgb() loader. This fixed the vertical stripes in the keograms, but made the fisheye and mapped images much darker (since the norm kwarg in plt.pcolormesh and plt.imshow does nothing).
• Refactored the TREx and Imager tests reflecting the minor changes.

Added

• A color_brighten kwarg to by default enhance the RGB colors when calling the following asilib.Imager methods, plot_fisheye, animate_fisheye_gen, plot_map, and animate_map_gen. Unless color_brighten=False, the plots remain the same.