Track/store reindexed trace ID (for each FOV or (FOV, trace group) value)

[vreuter]

Since the locus spots are viewed on a per-FOV or per-(FOV, trace group) basis, and since the Napari window sliders correspond to 0-based contiguous subsequences of integers, trace ID and locus timepoint get implicitly remapped into this range. This makes it difficult to refer back to the CSV-stored data backing the visualisation. This reindexed trace ID should be stored as an additional field/column in the CSV associated with each locus spot visualisation unit.