Remove digraph-parser dependency

[remikey]

In order to read the dot format, the DotTopologySerializer currently uses a dependency to com.paypal.digraph:digraph-parser:1.0 and thus to antlr.

We are considering the removal of the digraph-parser dependency; possibly by implementing the parser our self (using antlr, or something else).

[pictavien]

I started to write a new serializer based (only) on antlr.

[acasteigts]

That sounds good!