adding test file for top-down harmonization

[gidden]

This PR provides a notebook that is a first prototype for implementing top-down harmonization approaches.

It has a main operational function distribute_level() which tries to harmonize higher-dimensional trajectories (e.g., down to regions or sectors) consistent with lower-dimensional total (e.g., global total emissions).

The process begins by first using standard harmonization to harmonize global totals, and then attempts to successively harmonize higher-dimensional data. It is not perfect now and needs to be explored/played with to try to sharpen.

It may be that the best approach is not optimization-based at all but rather rule based, we have to see.

Required data files to run this are attached

harm_test_hist.csv.gz
harm_test_model.csv.gz

CC @coroa @chrisroadmap @znicholls @jkikstra