Grouping functions and capabilities

The old "OpenFormula" had level 1, 2, etc., and we
agreed that this was too constraining.  Interestingly enough,
the folks who were most concerned (wikiCalc) has since found
that it wasn't hard to add all the "level 1" functions, and
of course that aids portability.

Thus, for a start, here's what I propose:
* For every function, instead of saying "level X", simply
   give the function name as its (lowest-level) group.  If
   some capabilities are at a higher level (not everyone needs
   to support them), then append _2, _3, etc.
* Group the functions logically, and predefine a few levels
   in them the same way.
* Define something like "levels" at the end.

In the front, make it clear that an implementation need not
implement a particular level/group, but that the levels/groups
make it easier to explain what an implementation provides or
what a spreadsheet needs.

--- David A. Wheeler