Gop formula generation had a bug.

For graph  which vertices had  non trivial names (i.e.  different from
0,...,n-1) it would produce variables with the wrong name.

I also made the kth formula parser more robust.
Merge branch 'master' of github.com:MassimoLauria/cnfgen
Merge remote-tracking branch 'massimo/master' into master2