incorporate super-ecoregions into estimation workflow

Now calculate beta vectors for ecoregions and their respective super-ecoregions, per David's paper. Super-ecoregion beta vectors are only used for a given ecoregion if the share of Hansen hits to pixels in that ecoregion is less than `0.005` (i.e. very sparsely cleared during the training period). That parameter is [hardcoded](https://github.com/reddmetrics/forma-clj/blob/d6e17eca9b0a6ffa9d4c72ae71a0f45982b60ddc/src/clj/forma/source/ecoregion.clj#L20-23) into the `forma.source.ecoregion` namespace.

Addresses #390.