Bootstrapped modalities

Instead of just taking an event and assigning it a modality once, this resamples each event 100 times and takes the modality that gets assigned at least 60% of the time (can change the default via the parameter `thresh`).


```python
import flotilla

study = flotilla.embark('http://sauron.ucsd.edu/flotilla_projects/neural_diff_chr22/datapackage.json')
study.splicing.bootstrapped_modalities(n_iter=3)
```

Right now for some reason `study.splicing.bootstrapped_modalities?` doesn't give you the docstring :( This is because the result is `memoized` meaning it gets calculated once and then retrieved again, and the memoization wrapper thing takes over for some reason.