Add-in Furness balancing code

[Robinlovelace]

As discussed with @mem48.

[mem48]

@Robinlovelace see #36

[Robinlovelace]

Python code I found: https://github.com/SadraDaneshvar/Furness_Method

Interestingly the Python package spint does not mention it, and was last updated 4 years ago https://github.com/search?q=repo%3Apysal%2Fspint%20furness&type=code

[mem48]

Looks like the python code goes further than my PR as it does growth predicting. My code simply fills in an od matrix