[CHANGE] Mnanged directories

[Rob Beezer]

If you use the CLI, the following is likely of little interest. If you have a
very old project, like to use the pretext/pretext script, or are a developer,
you may want to skim the following.

* In the very old days we used "xsltproc" and you were in charge of directories
of extra material. But you could use a "directory.images" parameter to
customize some of that. That procedure is pretty much obsolete, and now it is
has been removed. You may get warnings.

* WeBWorK "representations" was an early development and the location of
related files was done in what now looks like an ad-hoc fashion. That
specification still works, but it would be a good time to move away from that,

* The specification of a directory for your "external assets" just moved to
#docinfo a few days ago.

New Situation

- As an author your files you bring to a project, like photographs being a
great example, live in a directory of your choosing, and that is specified in
#docinfo. If you have *any* such files, you need to say where they are, we
can't do it for you. But the CLI should help you out here and will default to
the directory being named, oddly enough, "external/".

- As a publisher, you make a lot of files from source material, with PreFigure
diagrams being a great example. So a file like slope-field.svg generated from
a PreFigure description will land in a directory named, oddly enough,
"generated/". This is all managed for you, so the directory will be created and
managed as necessary. If you prefer a different directory, you can specify it
in the publisher file, but that is now optional.

- This should all make for a smoother experience all-around, some annoyances
have now been removed.

Questions and comments on -support or -dev. Thanks for your patience with these
changes.

Rob