A lot of work on showing connecting two layers so that points in one
can have timeseries from the other:

- Change Timeseries object so that it can contain multiple timeseries.

- Add an ExtraGraphLine possibility to AugmentedDatasource so that it
  can return multiple-valued timeseries.

- Create IdentifierMappings because the IDs of both layers may not
  match.

- Create ProximityMappings, automatically created IdentifierMappings based
  on matching locations to the closests locations in the other layer.

- Call a script to create these from the admin interface.

- Give nicknames to DatasourceLayers, and only choose from nicknames
  when configuring ColorFromLatestValue, PercentileLayer or
  ExtraGraphLine options.

- Fix a bug in running the cache_latest_values script -- the stored datetime
  at which the script last ran was stored timezone-aware, but the calculations
  weren't, which led to an exception.

- Only create proximity mappings if a match is found within some
  maximum distance.
Preparing release 0.7
Back to development: 0.8