Filter to shapefile

### What problem does your feature request solve?

While selecting an area based on latitude and longitude (#400) is a good start, it does not cater for comparisons of more intricate regions, whether defined by orographic bounds or administrative ones.

### Describe the solution you'd like

It would be good to allow defining regions with some flexible format such as shapefiles or something.

### Describe alternatives you've considered

Start by reviewing the existing latitude-and-longitude area selection described in issue #400. Define how shapefiles or another flexible region format should be supported; done means users can compare weather and climate model data for intricate orographic or administrative regions.

Written by the indexing model from the issue text.