Modify gitignore to respect dot files in /files_for_testing

Merge branch '45-implement-property-system-for-nodes' into '19-implement-save-restore-of-opticscenery'

Resolve "Implement property system for nodes"

See merge request !11

Merge branch '44-allow-graph-vizualization-also-to-be-left-right-and-not-only-top-bottom' into 'main'

Resolve "allow graph vizualization also to be Left-> Right and not only Top->Bottom"

Closes #44

See merge request !10

added to_dot_test

Resolve "Write text for THRILL page"

Closes #38

See merge request !9

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.