Add alternate inputs for Dendrogram selector

[Seth-Rothschild]

• Henchman version: 0.0.3

Description

Currently functions take step as an input, which has internal meaning to the function (e.g. it's the location of the element in the self.edges and self.graphs lists) but is less important to the user than the threshold or n_feats.

Because there's a conversion from any of those inputs to another it is relatively easy to implement, but it's a difficult API question. Plan was initially to address this in #3 but got triaged.