More updates to CRS-related best practices

Hi. I've now updated BP3 [1] which is renamed:

*Best Practice 3: Choose the coordinate reference system to suit your
user's applications*
There are a bunch of commits in the Pull Request [2] (which is merged
already)-

* fixing a couple of typos in BP17
* formatting of long coordinate pos lists (BP17)
* moving useful text from BP3 to CRS introduction
* adding "evidence" requirement to BP17
* adding fourth method to specify CRS to BP17
* changed BP3 id and label
* significant update to BP3

The most notable commit is the last one. The commit comment says:

significant update to BP3

… focusing on helping publishers choose which CRS they should use - or
actually, when they need something other than WGS84.

Other minor editorial.

Also updated the requirements reference for BP17 back to the UCR doc.


As ever, feedback is welcomed. I think that we're getting there on the CRS
side of things.

BR, Jeremy


[1]: http://w3c.github.io/sdw/bp/#bp-crs-choice
[2]: https://github.com/w3c/sdw/pull/596