Write PreFigure images into the right default location

[rbeezer]

I think PreFigure images created with the pretext/pretext script are just going into the current working directory. In any event, they need to default to a prefigure directory of the directory for generated images.

• get_destination_directory() needs to be extended so the prefigure component goes into the prefigure directory (obvious addition to a dictionary).

• The dest_dir argument of prefigure_conversion needs to be respected. For example, the creation of the source files does this.

Pinging @davidaustinm.

[rbeezer]

I take it back. Without a definition of the "standard" subdirectory of the "generated" directory, the default is the current working directory. So the fix is a one-line addition that I can do easily myself and keep moving forward.

[rbeezer]

Done at c72ea632994194723e89a12af4b1e91d8e6e92e6