mshumko/aurora-asi-lib: Finished tests

(Not counting the example page in docs.) The following asilib.Imager methods are now tested: plot_fisheye(), plot_map(), animate_fisheye(), animate_map(), plot_keogram(), iter_files(), __getitem__, __str__, and __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