mshumko/aurora-asi-lib: Calculate geodetic skymaps and RGB auroral intensity during conjunctions

[0.20.0] - 2023-08-20 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