Track/store the reindexed (0-based) locus timepoints (per FOV or (FOV, trace group) combination)

[vreuter]

Since the Napari slider corresponding to the locus-specific timepoints is integers starting from 0, the notion of "which timepoint am I looking at" is somewhat lost. It would be helpful to store this information as an extra column in the CSV files associated with this visualisation. Each row corresponds to a specific voxel, and each voxel is a single viewing unit (modulo the $z$ slider), so store this value (the reindexed locus time) per such row. Specifically, for a fixed position of the bottom slider (reindexed trace ID) each row in a CSV associated with the visualisation of the locus spots corresponds to a fixed position of the middle slider (locus timepoint), and all values over the top ($z$ slider).