Complete the TeX, HTML and RTF printing functions.

Summary stats: don't bomb out when one variable is all missing,
if there are variables which are not all missing?

Test: use of addto/omitfrom in console, scripts and GUI

Long-term: make model_sample_check smarter -- don't just
disallow commands, but temporarily adjust the data set to
make them possible.  Maybe?  Probably just too complicated.

Resetting sample range.  Needs testing.  Also some issues:
* Any way to preserve "structure"?
Also need to update hlp files and manual on this.


