Adding arrows to panel PCA plots

[leesharkey]

So that we can see hx path easily. There are some figures early in the Slack channel that connect consecutive hx points within a single sample. That's what we want here. It makes understanding the trajectory easier at a glance.

[leesharkey]

Decreased the priority of this to NotImportant because the lines are good enough. If it's easy to put arrow heads on, then go for it, but the project can be published without it, hence the demotion.