Celltype order

- [x] Add violinplots to PCA outputs
- [x] Make it easy to plot a single splicing event's psi
- [ ] Make it easy to plot a single gene's expression
- [ ] `Study.get_feature_subsets` not working:
    - [ ] Interactive_graph (https://github.com/YeoLab/flotilla/issues/86)
    - [ ] Interactive_classifier (https://github.com/YeoLab/flotilla/issues/87)