[DEPRECATE] Short-lived "datafile" element

[Rob Beezer]

The "datafile" element was born last November 4. It is going to be supplanted
by a new and improved version, powered by Runestone Javascript (and necessary
for a currently running course!). Perhaps the shortest-lived element ever. If
you don't use "datafile" you can ignore the rest.

But the old version is still a useful construction, so we have implemented a
cosmetic replacement, "dataurl", which might actually be a better name anyway.

Pre-processor will handle the change. But perhaps for a limited time only - it
will become hard (or impossible) to distinguish old from new once the latter
comes into being, so no promises. So not an immediate change, but perhaps best
to make this one a priority.

[Rob Beezer]

As suggested, this deprecation got a bit messier. The new "datafile", just
announced, requires the use of a @label attribute. So absent that, we assume it
is an "old" use.

"datafile" w/ @label - new feature, full-steam ahead

"datafile" w/out @label - old use, warning, internal cosmetic upgrade to new
"dataurl"

Short version: edit your source to convert any old uses of "datafile" and make
them into "dataurl", before your next CLI upgrade. Not too rough.

Rob