Plot PC axis interpolation path in UMAP visualization

[zhonge]

I've gotten the above feature request for figure-making purposes. I think it would be nice additional visualization to show the PC interpolation path in the pc*/umap.png plot produced by cryodrgn analyze.

Since the PC trajectories are never embedded with UMAP, there are two options for how we can visualize the path:

1. Plot nearest data embeddings (fast). Maybe we can include an additional warning if the nearest data point is far from the PC axis.
2. Run or rerun UMAP with additional PC axis trajectory points (slow)