add Beale's ratio estimator as its own model

not yet sure this is a great idea, because (1) BRE only operates at annual to multi-year timescales and thus ignores a lot of loadflex functionality, and (2) the BRE method is getting developed further this year, so it might be better to wait. still, worth considering b/c BRE has great performance at sites without trends when the target is multi-year means

The issue names no files, tests, or entry points. Start by reviewing the existing estimator models and the Beale's ratio estimator's current development and timescale limitations. Done would require a decided scope for supporting BRE as its own model, including how it fits the existing loadflex functionality.

Written by the indexing model from the issue text.