Display more than one probability density in TrackPositionAnimationV1

[khl02007]

It would be good to be able to show two different decoded positions in the same view (using two different color maps) so that one could compare the two decodes more easily.

Relevant place:
https://github.com/magland/sortingview/blob/main/sortingview/views/franklab/TrackPositionAnimationV1.py