"tabf", new keyword

[David W. Wilson]

IMHO, the "tabl", "tabf", "tabh" approach to table formatting is the wrong approach.
I propose rather a change to the indexing which would not only make tables more
general and easy to manage, but would also help with non-tabular sequence indexing.

Here is my proposal:  Redefine the %O line or introduce a new indexing line which
allows the user to specify one or more sequences as an indexing sequence (here I
will assume the %O line is replaced).  If one sequence is specified