added a module to report basic infos on petri nets
small changes to gcd code

Corrected mostly spelling mistakes.
Added optimisation: When computing the gcd of more than two
  numbers, return 1 as soon as the intermediate value is 1
  instead of performing more (pointless) gcd calculations.
Merge branch 'master' of github.com:renke/apt
added reporting of labels to info module

The Petri net info command now also reports the number
of different transition labels, so the user can quickly
see if the net is injectively labelled.