Add scale bar to bounding box maps in dataset documentation

[pont-us]

Currently, the datasetdocgen.py script produces maps with a grid and longitude/latitude labels, but no scale bar. In the discussion on #3, @AliceBalfanz suggested adding a scale bar. I think this is a good idea but it's not entirely trivial to implement: currently the map uses a Mercator projection, so the horizontal scale varies significantly with latitude and we therefore can't produce a scale bar which will apply to a whole plot. If we're going to have a scale bar we should instead use projections which minimize distance error -- maybe a locally centred Lambert azimuthal equal-area for local/regional data sets, and Mollweide for any bounding box big enough to be shown on a map of the whole Earth. Cartopy supports both of these. Dropping Mercator is probably a good idea in any case because of the notorious vertical distortion at high latitudes.

[pont-us]

Commit 323a57d switched from Mercator to LAEA, so the remaining tasks are:

1. Decide what to do about very large (up to global) areas, since LAEA isn't very good for these.
2. Implement whatever was decided.
3. Actually add the scale bars to the plots.

Items 1 and 2 could be postponed until we actually have a dataset which requires them.