Proposal: rename "about" identifiers on PlaceDescription / SourceDescription

Hi,

As previously discussed in #79, I find the "about" identifiers to be unclear field names.

Currently, it is in use in two places:

* **SourceDescription**: `about` is "a uniform resource identifier (URI) for the resource being described."

* **PlaceDescription**: `about` is "a uniform resource identifier (URI) for the place being described. This can be used for associating descriptions of the same place."

As I understand, these fields do not carry the same meaning, and should have different names - or their meanings clarified.

I am a little unclear on use-cases for `SourceDescription.about`, but I think it is to link the description to a given entity in the data. That is, "this source description and its extracted conclusions describes the entity in its `about`". Is this correct?

I mentioned in #79 that I think `PlaceDescription.about` in its current incarnation would probably be better called `placeID` or similar to reflect its meaning.