[Tagging] access in the wiki: move psv to "by use"

[martinq]

> I started mapping in Jan 2008. By that time it was already clear that
> "psv" was taxis and buses. If we start questioning every consensus (even
> those documented on central pages of the wiki like the access-page) we
> can stop mapping now ;-)

I have interpreted psv (public service VEHICLE), bus and taxi as vehicle 
categories in the past, but never required these keys in my area.
So for me an empty taxi is allowed on taxi=yes.

There are two issues, nobody has probably paid attention on so far:
1) "public service" is not "public transport", as intended by the 
creators of the key. So if people make a road cleaning truck or an 
ambulance a PSV, then this was maybe not intended, but a result of 
ambiguous documentation/naming.

2) Taxi is actually not really a means of public transport (maybe 
disputed, I am sure several definitions of PT exist). The inclusion of 
taxi supports the misunderstanding that the "psv" key means a broad 
range of public service vehicles (road cleaning, etc.).

--

Not sure if we can fix this misunderstanding (turn restriction plug-in) 
retrospectively in our database. Probably "psv" is broken now.
But I do not see any real need for it. It is just coincidental that 
taxis can use bus lanes in some countries, I do not see the need to 
create a hierarchy just for this purpose. We can tag it with "taxi" and 
"bus" separately, "PSV" (or "PTV") is a rather artificial group.

--

Also the approach to mix use (public service/transport) with vehicle 
type was probably not the best choice back in the early days. It created 
the weird issue of requiring a new category for buses not used for 
public transport, since "orthogonal" use and type cannot be freely combined.

For my suggestion to use key/value for category/use, e.g. bus=yes (any 
registered bus), bus=public_transport (only buses used for 
public_transport) [see other post] it is probably too late, even though 
the use of bus as access-key (and not as public_transport key) might be 
limited.

Better backward compatibility I refine my proposal in the other post a 
little bit:
1) Update key hierarchy:
+ omnibus  Vehicle registered as bus
+++ bus    Omnibus vehicle, used for public transport at point of access
2) Introduce value "public_transport"
omnibus=no & bus=yes can also be expressed as omnibus=public_transport
3) Depreciate"psv" (or broaden the meaning to all "public service" vehicles)
4) Depreciate "tourist_bus": There is no longer the need for tagging 
both ("bus=yes" and "tourist_bus=yes") in the case any bus category is 
meant. It can be expressed by "omnibus=yes" now.

martinq