Resolve trace item locations dynamically from the graph

[rsimon]

Example:

• the TEI links to an item that's represented in a Linked Traces dataset (e.g. an archaeological artefact)
• the item has no location of its own, but links to a place in a Linked Places gazetteer

At the moment, Peripleo does not show a location on the map, if an annotation in the TEI links to the artefact (because it's not a place). In this case, the location should be derived on-the-fly, by walking the graph, following up connections to the nearest place the artefact links to. (In practice, we can probably restrict this walk to just the nearest neighbours, assuming that trace items will generally be linked to a place.)