[BP] Updates to BP6

Hi all,

Just calling for a review of BP6 - [1] Describe properties that change over time

First, I changed the title. It was one of those BPs that still had a title starting with "How to". I wasn't happy with that, but am not 100% happy with what I changed it to, either. Suggestions welcome!

Second, I added two code snippet examples.

The first one illustrates possible approach 2) (providing a series of immutable snapshots). The example was taken from gemeentegeschiedenis.nl which publishes historic and current boundaries of municipalities. However I changed the GeoJSON I got from there to reflect our recommendation. The original code has all historic as well as the current boundary in one GeoJSON file. I changed this to multiple code snippets referring to one another. I asked the developer of gemeentegeschiedenis.nl for a review as well.

The second one illustrates possible approach 3) (capturing a time-series of data values). For this I used a GPX file showing the changing position of a runner.

Linda

[1]: http://w3c.github.io/sdw/bp/#desc-changing-properties