gt/gnmgame: add functionality for symmetric games

merged from agg.cs.ubc.ca code
gt/gnm: diagnostic outputs & fix div by zero

merge from agg.cs.ubc.ca code
Fixed comments for gameagg
gambit-gnm support for AGGs

Merged in wrapper class aggame from agg.cs.ubc.ca code.
Refactored gnm-specific code from libagg/agg to gt/aggame
gambit-ipa support for AGGs