refactor: add flexibility to plot_upgrades_map

[danielolsen]

Pull Request doc

Purpose

Allow transmission upgrades plots to be created using any interconnection, and to properly plot new branches as well as scaled branches.
Closes #267.

What the code is doing

In upgrades.py: we fix the merging of the two branch data frames to ensure that the lat/lon columns of the merged data frame include the lat/lon data of new branches.

In plot_upgrades_map.py:

• We make use of the plot_states function to simplify border plotting.
• Previously-hardcoded b2b_indices are now passed by the user.
• Previously-hardcoded figure sizes can now be passed by the user (default sizing remains, which is good for plotting USA).
• The user can specify explicit x_range and y_range, to be able to zoom into certain regions.
• Scaling factors are now specified in pixels/GW, not pixels/MW, based on conversations in feat: improve flexibility of utilization, risk, and bind maps #266.

Testing

@danlivengood has been using this code to plot maps of Eastern scenarios with new branches.

Time estimate

10 minutes. The data frame merge is the most complex part, most everything else is just adding flexibility to previously-hardcoded values.

[BainanXia]

The logic makes sense. Given Research Dan has been testing this for multiple times, I believe this is good to go.

[rouille]

Thanks