Feat: Added new data structure to the Markers that allows to lookup the referencing Markers of child Sequences + fixed panic

[hmoog]

Description of change

To be able to traverse the Sequence DAG in both directions, we introduced a new data structure that allows us to query the referencing Markers.

At the same time, this PR fixes a panic that appeared in some rare edge casses.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)

Change checklist

• My code follows the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes