Handle keyboard interrupt from python

[johnlees]

Need to add a check into the sketch and dist loops. See:
https://pybind11.readthedocs.io/en/stable/faq.html#how-can-i-properly-handle-ctrl-c-in-long-running-functions

[johnlees]

While doing this, I should also handle the throw when there's no sequence properly (the current crash it causes isn't particularly consistent in whether it gives the problematic sample)