added to_dot_test

not really useful yet...

Scenery::connect_nodes return () instead of EdgeIndex since it's not
needed anaywhere.

Fixed tests for group. All tests pass again.

Remove example trait_test.
Doc fixes.

Optical::name returns node type as default.

Source: add name parameter.

This forces the usage of the "new" function to submit a name.

Update examples accordingly

Separate Dottable trait to additional file.

Add unit tests.

The original graph is kept untouched.