Satsim

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* Add support for piecewise rendering. Set sim option `render_size` to enable. For example, [256, 256].
* Add `fixed` tracking mode with mount azimuth and elevation.
* Add great circle propagator for targets.
* Add in-memory image generation. See generator function `image_generator`.
* Fix missing stars when FOV crosses zero degree RA.
* Add curved targets using bezier curve raster. Enabled by default. Set sim option `num_target_samples` to 2 to enable linear raster.
* Add LRU cache to star catalog reader.
* Add option to turn off SNR calculation. Set sim option `calculate_snr` to false will render targets and stars together.
* Handle unstable SGP4 TLEs.
* Add TensorFlow 2.4 Docker build file.
* Add debug output for pristine images of targets and stars.

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* Calculate POPPY input wavefront resolution to avoid PSF aliasing.
* Add support for additional FITS image data types (`int16`, `uint16`, `int32`, `uint32`, `float32`).
* Add batch processing to `transform_and_add_counts` to support batch processing of stars.
* Add `auto` option to calculate temporal oversample factor based on star velocities.
* Add option to turn off serializing config data to pickle file (`save_pickle`).
* Add option to turn off png movie output (`save_movie`).
* Add `crop_and_resize` and `flip` image augmentation.
* Set pixels with values beyond the pixel data type's capacity to the maximum value for that data type.
* Add `lognormal` function to generate a distribution with a true target mean.
* Fix issue with sidereal track.
* Fix issue with fragment velocity not being randomly sampled.

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* Add Physical Optics Propagation in Python (POPPY) PSF generation.
* Add PSF augmentation with `$pipeline` replacement key.
* Add `$function` and `$compound` replacement key.
* Add ability to generate stray light from a `$function` replacement key.
* Add built-in 2D polynomial image generator for stray light, `polygrid2d`.
* Add built-in cosine fourth image generator for irradiance falloff, `radial_cos2d`.
* Add built-in sine wave image generator for fix pattern noise, `sin2d`.
* Add built-in image generator from AstroPy model, `astropy_model2d`.
* Add built-in image augmentation, `scatter_shift` and `scatter_shift_polar`.
* Add `$cache` replacement key (caching works for PSF and `$function`).

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* Fix new Skyfield incompatibility.

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* Prefix replacement keys with `$` in SatSim configuration file.
* Add option to scale collision fragments by cosine of the exit angle.

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* Add astrometric metadata into FITS header
* Refactor WCS library
* Add option to flip images about x or y axis
* Add option to refresh stars for each frame
* Add RPO from TLE generator