Until now, we checked the suffix of the specification path to determine whether or not to interpret it as a graphml automaton or not. Instead, we now check the contents of the file and iff they are XML data, we assume it is a graphml automaton.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13805 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c