Add in option to plot results on a sub-region basis

[kmongird]

Is your feature request related to a problem?

When using cerf.plot_siting() to show a map of locations of individual power plant results, it would be beneficial to have the option to show the map on an individual sub-region basis (e.g., state) rather than show the entire region (e.g., US)

Describe the solution you'd like

Add in a parameter to cerf.plot_siting() to specify which sub-region to plot or a parameter to only plot sub-regions included in the output dataframe.