fix(find_overlap): missing right study type in output

[DSuveges]

✨ Context

There was a recent change that studyType in identifying study locus overlaps no longer comes from study index, instead of studyLocus. It came with a side effect that upon doing outer join in the _align_overlapping_tags method made the rightStudyType nulls for tags where the left tag was not found in the right locus. This causes a range of problems downstream.

Scope of this PR

• Apply a fix in the .find_overlaps() method to carry over rightStudyType from the _overlapping_peaks method.
• Adding sample dataset of two study loci for testing.
• Adding tests to demonstrate behaviour.