Asilib

- Changed all instances of the word `station` and replaced it with `location_code`.

- The main change is the `ignore_missing_files` parameter that is passed to the download functions. When True, the `download_themis_img` and `download_rego_img` will not raise a missing file error when data from that hour does not exist.

- The biggest API change is the parameter order for `download_rego_img` and `download_themis_img`. Now it is `(location code (i.e. station), time, and time_range)`. Beware that now the parameter order API is inconsistent across all of the functions---I will standardize it to (`asi_array_code`, `location code (i.e. station)`, `time`, and `time_range`) in the next minor (0.X.0 release).

- For consistency, I removed most instances of the word "frame" and changed them to "image". This propagated to the following function renaming (deprecation of the old name).
- Deprecated the get_frame and get_frames functions for load_image. It is a wrapper for _load_image and _load_images functions that were once get_frame and get_frames. I added this function to standardize the load/download names. It returns either one or multiple images, depending on if the time or time_range keyword arguments are given; it will raise an error unless time or time_range is passed (not both).
- Renamed the plot_frame function to `plot_image`; plot_frame is now deprecated.

- Removed deprecated functions