[Nauty] nauty&Traces version 2.6r7 released

[Brendan McKay]

Version 2.6r7 is available.  Changes:

1. Documentation of the delptg tool was missing from the manual.

2. listg -d (write in dreadnaut format) did not emit the "d" command for 
digraphs.

3. A technical fix was made to testg.c. If you don't write your own 
pick/count code,
     this didn't affect you.

Thanks to Jörgen Backelin for reporting 1 and 3.

Brendan.