Fix Bug in tracksMerger

[servantftransperfect]

This pull request updates the logic for merging tracks in the TracksMerger::addTrackMap method, with a focus on improving how features are added and merged when tracks overlap. The changes clarify the merging process and ensure that feature replacement is handled more intelligently.

Track merging logic improvements:

• Added comments to clarify the process of looping over input tracks and the rationale for merging tracks if features already exist, making the code easier to understand and maintain.
• Updated the feature replacement logic to only replace an existing feature in a track if the input track is larger than the existing one, and added comments explaining this decision.

Feature tracking and updating:

• Moved the storage of feature information (TuplePoint in _existingTracks) to after the replacement logic and added explanatory comments, ensuring that the tracking of existing features is accurate and up-to-date.