extend IO with DOT format

[Meijuh]

For easy inspection of state spaces it would be useful to have dot output.

Example code can be found in pins2lts-seq. If implemented in IO-lib, the output format will be supported by all tools.

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Branch bergfi/dot-io at http://deathegg.student.utwente.nl/git/ltsmin.git does this. Uses record output of dot and shows state vector, state labels and a single edge label Some special labels: custom_dot: allows the front-end to customize dot output (both edge and state label) action: this edge label is printed as edge label error: 0: normal, 1: state is coloured red (state label) errormessage: char[] chunk containing an error message. If non-empty, is printed in the state (state label AND state vector variable)