ENH: Allow historic targets to be per-pixel OR per-polygon

[szwiep]

Issue

Current implementation of the tool only allows historic recovery targets to be computed on a per-pixel basis. Allowing users the choice between per-pixel and per-polygon provides additional flexibility and potential use-cases for users. Feature suggested by @melissabirch.

Proposed Feature

Provide parameter to allow users to determine their target method. Either a boolean or str input. This will likely be easier to develop after the API update that will be introduced in #34.