tmap_model.trajectories fails with two time points

[concatenize]

I'm excited to try out WOT on some public data! But, I ran into an obstacle when testing modifications of notebook 4 on a small subset of the data.

tmap_model.trajectories seems to only work when 3 or more time-points are present. You can verify this in notebook 4 by deleting all but the 17.5-18.0 map. The error with only these two time-points is ValueError: No cells found at the given day, which is confusing because there are plenty of 18.0 cells in the relevant Population object. It would be nice to have an informative error message when only two time points are present.

This may be related to #25 .

[joshua-gould]

Fixed bug when reading transport maps with only two time points