Petri net checker

I have added typechecking.{c,h} and modified the Makefile.am to compile mist with this two files.

There are no errors when compiling but I didn't test yet the functions
Efficiency is not a concern since we are just parsing the input file. Therefore, it is fine to browse all the tree even if you conclude from the first node that it is not a Petri net. As for the return value why not patch tree_dump to handle return values from the functions it invokes.
tree_dump is currently only invoked in two places:
mist.c
1286:	tree_dump(atree, callback_tree_before, callback_tree_after, callback_leaf);
transsystem.c
74:  tree_dump(tree, callback_tree_before, NULL, NULL);  
so it should be little work to patch it.