Implement plugins for Filters and Classifiers

[thequicksort]

I have code written for a "Plugin" architecture, that lets users/clients define their own filters and classifiers, then register them for use on the relevant Poretitioner steps

This opens up security concerns (e.g. what if a user writes a malicious plugin?). As long as all execution remains on their machine, this won't harm us, but we should still be congnizant of input santiziation, etc.

https://hackernoon.com/10-common-security-gotchas-in-python-and-how-to-avoid-them-e19fbe265e03