[Tagging] waterway=fish_pass consistency

[Richard]

On Thu, Jul 19, 2018 at 11:16:57AM +0200, Yves wrote:
> You'd be *very* surprised what shapes fish passes can take.
> I don't think it's a good idea to use the waterway key to tell that whatever feature is intended to be a fish pass. 
> This one, although man made is definitely a river, that was built as 1) a fish pass, 2} a whitewater course. 
> https://www.openstreetmap.org/#map=17/45.84919/5.41420

indeed and as they are getting more widespread more variations can be
expected.

Recently a small river nearby was converted to a fishpass where a weir
was previously. Thus it is definitely a river and a fishpass at the
same time and I have seen kayakers going down there.

So using a particular watervay value is no longer fitting but at the same 
time I don't like using more and more top-level keys to describe particular
properties of waterways if there is no reason to believe that for example
the fish_ass property will be ever useful to any non-waterway object.

Richard