Asilib

- Updated GitHub Actions:
- test the `cartopy` maps,
- test using `ubuntu-latest`.

- The following `asilib.Imager` methods are now tested:
1. `plot_fisheye()`,
2. `plot_map()`,
3. `animate_fisheye()`,
4. `animate_map()`,
5. `plot_keogram()`,
6. `iter_files`,
7. `__getitem__`,
8. `__str__`, and
9. `__repr__`.

- The `asilib.themis()` and `asilib.rego()` examples are now tested as well.

- Fixed a `asilib.Imager` bug that was triggered when the time slicing is outside of the time_range. Before, it raised an unhelpful `AssertionError`, but now it raises an informative `FileNotFoundError`.

- Added a `asilib.asi.fake_asi` function to quickly test `asilib.Imager`.
- Using the `fake_asi` I found and fixed a few errors in `asilib.Imager`.
- Added more tests for `asilib.asi.themis()`, `asilib.map.create_map()` and began adding tests for `asilib.Imager`.

- Added tests for `asilib.themis()` plotting functions
- Moved map creating examples to `asilib.map.create_map()`.
- Fixed numerous minor bugs in `asilib.Imager()`.

- Added `Imager.plot_map()`, `Imager.animate_map()`, `Imager.animate_map_gen()` methods and documentation that project images, or a series of images, onto a geographic map.
- Added `Imager.keogram()`, and `Imager.plot_keogram()` methods to make keograms
- Along the meridian or a custom path
- With geographic or magnetic latitudes, depending on the `aacgm` kwarg.
- `asilib.Imager` functionality is complete, with a first draft of the documentation & examples.

- Added an Active Development warning in the `asilib` docs.
- Simplified the `Downloader` class.
- Fixed a download bug. It arose because the `overwrite` kwarg played two overlapping roles: to redownload data and overwrite animations (the ffmpeg argument). I fixed the bug with the `redownload` kwarg reserved only for downloading data, and `overwrite` kwarg reserved for overwriting animations.
- Added cardinal directions to `asilib.imager.plot_fisheye()` and `asilib.imager.plot_fisheye_gen()` methods. To use it, set the `cardinal_directions` kwarg to one or more directions, e.g., 'NE' for north and east directions (default), or 'NEWS' for all directions.
- Improved the `asilib.Downloader` class.
- Replaced the `_check_url_status()` implementation that checked if the server is online. Turns out, if the url argument includes a path to a large file, `asilib.Downloader` will download the file just to check the server status. I replaced it with `request.raise_for_status()` that doesn't download the file.
- Added a try-catch block for streaming large files. This addresses the bug where a file is partially-downloaded if the stream is interrupted. This goes unnoticed until `asilib` raises file-corrupted errors that are hard to track down. The fix first deletes the partially-downloaded file if the stream is interrupted, and then raises the error.
- Added functions, documentaion, and examples for `asilib.map.create_map()`, `asilib.map.create_cartopy_map()`, and `asilib.map.create_simple_map()` to use with `asilib.Imager`.