Make TF-IDF a function for similarity anchor instead of its own anchor type

[WarmCyan]

ICAT could come with a small library of functions that could be quickly passed into the model.

This would require re-thinking how the "example" button works, as we'd need to have a default/a way to set what similarity function should be used by default.

[WarmCyan]

Instead I decided to go the complete opposite direction and instead of having similarity functions, make it so similarity functions have to subclass the anchor, see #39