Remove Exporter from Graph and Graph::__toString()

[clue]

This PR removes all references of the Exporter from the main Graph class, and hence its __toString() method, some reasons being:

• This library will be split into smaller components (Split off individual components, stabilize core graph lib #120)
• The Exporter namespace will be split off into a separate package (Split off trivial graph format (TGF) exporter #121).
• Also, see Edge has no __toString() #23 for details why __toString() is to be avoided.

[clue]

Updated this PR to only "remove Exporter from Graph and Graph::__toString()". I.e. it no longer removes the whole Exporter namespace, this will be done in an upcoming PR.