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
To check, you count syntactic elements, therefore X' = Y + 1 would be accepted as a Petri net, right? This is not correct. The same variable should occur on both side of the equality.