treatment of None arguments helps deal with 1-plot case
more care with treatment of None values, to support having just one dataset.
The none_ok stuff is not used after all, so I'll take it out for the next commit.
We don't really need a special provision for keeping track of inputs where None is ok - at least, not yet.
Treat "no filetable" as a real possibility, and not an error (it's
possible if you're plotting from one dataset, not comparing two).
Improve drange __repr__: don't print units if they don't exist.
Behave reasonabley when there are no files to be found, not even a
directory to look in.
Several changes to handle a nonexistent second dataset - e.g. the plot
can be a uvc_zero_plotspec rather than None.
better treatment of the no-data case
plot sets 2 & 4 can handle the case where there is only one dataset
not the usual two