[Nauty] foreshadowing a change to sparse6 format

[Brendan McKay]

* Gordon Royle <gordon at csse.uwa.edu.au> [050430 09:42]:
> Is there a default "internal format" for sparse graphs?

Yes, but I didn't tell anyone what it is ;-).
 
> If one uses readg() to read graphs, then it copes transparently with 
> either g6 or s6 as input, but the resulting graph pointer is to a 
> bit-packed adjacency matrix... at least, unless I have missed some 
> recent changes ...
 
I didn't distribute any of it yet.  Version 2.3 will have support for
a sparse format within nauty, but only a few of the utilities (such as
labelg) will support it.  There are routines read_sg(), writeg6_sg
and writes6_sg that read and write from the sparse format.  After
a while I'll ask for volunteers to test it.

Brendan.