feat: allow congested mesh branches to be ranked by mean congestion

[danielolsen]

Pull Request doc

Purpose

Allow congested mesh branches to be ranked by mean congestion, rather than only by a quantile value of congestion. Closes #439 (we will create a separate, more detailed issue for the third bullet of #439, which requires more design thought).

What the code is doing

Previously, we had a method parameter, which only applied to the denominator of the (congestion metric) / (upgrade cost proxy) ratio. Now, we have congestion_metric and cost_metric (replaces method). Most of the code changes are simply renaming method -> cost_metric, and elaborating in the documentation. Within _identify_mesh_branch_upgrades, we call .mean() rather than .quantile() on the congestion data frame as applicable, and we rename some variables so that we create a congestion_metric_values variable regardless of whether it's a quantile or a mean (previously, this variable had been called quantile_cong_abs).

Testing

Previous unit tests pass, new ones have been added which return different results if we select "mean" rather than "quantile" as the congestion_metric.

Time estimate

15-30 minutes. Functional code changes are <10 lines, but a few other things are renamed or reorganized for clarity.

[BainanXia]

Looks good. Thanks!