Consolidate file format-related configs

Enhancement proposal

Currently there are a number of configs that relate to the output format:

--format
write_to_movie
movie_file_extension

I think there is redundancy here and they are all being handled in different ways, meaning there is room for contradiction. E.g. if config such as --format is set to png and write_to_movie is set to True, or movie_file_extension is gif and --format is mp4. Then you need to set up some precedence and it creates confusion.

I propose to only use a single argument to represent the output format and think --format would be the most suitable. We have other considerations such as if the t flag is set then it may override --format, but cases like this would be easy to handle if we only need to think about one argument. We can determine if we "should write to movie" and what the file extension should be based on the format.

Additional comments

Trace how --format, write_to_movie, and movie_file_extension are parsed and used, then compare their behavior for conflicting combinations such as PNG with movie output or MP4 with a GIF extension. Done means the output format is represented consistently by one argument and the related movie-writing and extension behavior no longer requires conflicting settings.