Plot in scores alongside plot that shows location (map) of which obs have been used

### What problem does your feature request solve?
Following approach of RES, if we are aggregating verification stats across multiple case studies, we only include stations where coverage is continuous through period. Thus, for longer aggregations, we might lose a few stations. It would be good generally to know what stations are going into an domain average, aggregated metric.

### Describe the solution you'd like
A plot generated under the same category in CSET that displays station coverage (points on a map).

### Describe alternatives you've considered
Not knowing what your verification includes (not good QA).

Start by tracing how existing scores plots are generated in CSET and how continuous station coverage is selected during aggregation. Add a station-coverage map under the same category, showing which stations contribute to the aggregated metric; verify it using an aggregation that excludes stations with incomplete coverage.

Written by the indexing model from the issue text.