Bounding Polys of data.available

[bstrdsmkr]

Is your feature request related to a problem? Please describe.
Often, areas of interest lie close to the edges of the available pbf downloads, but it isn't always clear if it's necessary to download adjacent or superset datasets.

Describe the solution you'd like
Add "bounding_polygon" as a property of the data.available classes. This could be calculated once and allow users to make better decisions about what to download, saving the users time and perhaps more importantly, lightening the load on the backing data services.

This is also somewhat related to the idea of providing subsetting and saving functions, though not a total solution there, it may help solve some sub cases