Allow users to restrict annotations to a bounding box

[rwood-97]

Is your feature request related to a problem? Please describe. This idea is that users could load a whole corpus of maps (e.g. the UK) into MapReader's loader but then restrict annotations to a bounding box (e.g. a small area of scotland). This would enable users to annotate key areas of their maps more easily. At the moment, patches are either shown in random order or using the sortby keyword to sort by a df column. PR #319 also adds the filter_for kwarg that might enable users to do this kind of filtering for bounding box but would need documenting.

See #173 for further discussion .

Describe the solution you'd like Either:

• Add ability to filter for bounding box (as kwarg)
• Add docs for how to filter for bounding box using currently implemented features