updates to BP17 (coordinate reference systems)

Hi.

I've been working on the CRS material in the best practice document to make
sure that the introductory material and each of the two CRS-related best
practices are clear and don't overlap.

Updates to the intro were largely editorial.

But BP17 has had a significant update. I've merged it into the Editor's
Draft [1], but see this commit [2] for details.

The commit comment reads:

```

Refocus BP17 on “State how coordinate values are encoded”, making the

subject matter clear about how a publisher can provide enough

information about how coordinate values are encoded so that a user

isn’t left with any ambiguity.

Overlap with BP3 and CRS intro material now removed.

I hope the examples are right!
```

The last comment is a plea for someone to review the examples I've
included. They're hand-written, so may include errors. Also, it being the
weekend, it seems my brain has gone to sleep - so I might not have written
my JSON-LD (etc.) correctly :)

Lastly, (and this should be amusing seeing as I'm writing a BP that says
"get your CRS axes the right way around") can you WG members check whether
I've got latitude and longitude the right way around in the examples.

I think:

- GeoJSON (based on OGC CRS84) is Long/Lat
- schema/GeoShape is Lat/Long
- EPSG:4326 is Lat/Long.

Next up is a spring-clean for BP3 (the other CRS best practice) so that it
discusses "how to choose the right CRS" and basically say: WGS84 _and_ what
ever else your users are yelling for.

BR, Jeremy

PS: Kudos to Andrea - I've repurposed some of his fine examples with
geometries of Anne Frank's House!

[1]: http://w3c.github.io/sdw/bp/#bp-crs
[2]:
https://github.com/w3c/sdw/pull/595/commits/456025a434e9ce05a3119191b0ee2eca9cba9019