Mist with the new check for petri nets works fine

I have also updated the Copyright
modified tree_dump to can use this function in is_petri_net. I have also modified all the ocurrences of tree_dump and call_back functions used with it
I think this is clearer
Fixed 'possible_petri_net' bug and some errors from my last commit
fixed the bug about the possible/not possible constant values for the actions of a petri net.