Asilib

Added
- `custom_alt` kwarg to the THEMIS, REGO, and TREx loaders. Credit: Cassandra M.
- A test for the `custom_alt` functionality.
- Tests for the custom colors.

Changed
- How RGB color channels are loaded. By picking one or multiple color channels, the underlying data for the unselected channels is masked as NaNs. matplotlib handles these images well.

Added
- An advertisement figure and script. The script in `examples/global_coverage_map.py` plots a geographic map showing the spatial coverage (low-elevation field of view rings) of all imagers supported by aurora-asi-lib. The resulting plot is located in `docs/_static/global_coverage.png` and is shown in the `README.md` and `index.rst` files.

Fixed
- A bug with TREx RGB which resulted in a `ValueError: A problematic PGM file…` error when new data files are downloaded. I added a Warning block to the documentation to instruct users to update asilib

Changed
- Incremented the minimum `trex-imager-readfile` version to 1.5.1 to work with the updated TREx-RGB image files.

Fixed
- A bug in `Imager._calc_cardinal_direction()` method that manifested in non-orthogonal directions when the ASI (az, el) skymaps are offset such that the low elevations are outside of the field of view.
- A bug in the REGO example.

Changed
- Shortened the namespace paths in the Imager API Reference page. Their namespace is the same as it would be imported.

Added
- RGB auroral intensities in `asilib.Conjunction.intensity()`.
- An auroral intensity test (from the nearest pixel and equal area) using the TREx-RGB data.
- `asilib.skymap.geodetic.skymap()` function that maps the (az, el) skymaps to (lat, lon) skymaps assuming Earth is a sphere (i.e., not an ellipsoid).
- Added a plot test comparing the official and the asilib THEMIS GILL (lat, lon) skymaps.

Changed
- The `asilib.Conjunction()` class had am ambiguity regarding whether if the satellite ephemeris was interpolated (or downsampled to) the ASI time stamps, or kept at the original cadence. This ambiguity made calculating auroral intensity error-prone, so now `asilib.Conjunction.intensity()` automatically interpolates the satellite ephemeris.

Fixed
- A bug in the asilib.Imager.data property that relied on hard-coded filtering of unfilled images using `np.where()`. This led to duplicate time stamps for RGB images, and a crash with `asilib.Conjunction.intensity()`.
- Removed the THEMIS and REGO imports at the top-level of asilib, i.e., in `asilib/__init__.py`. Now they are imported in `asilib/asi/__init__.py`, so users must always import asilib.asi when using ASI modules contained within the `asi/` folder.

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.

Added
- `asilib.Imagers.get_points()` method to get the (lon, lat) and pixel intensity points.
- Tests for `asilib.Imagers.get_points()`