Warn about using leise=park on nodes

[mdk]

According to the wiki, leisure=park should not be used on nodes (and ways, only on closed ways). JOSM shows (correctly) the "unknown" icon. But the walidator shows no warning or info.
In fact it is used over 55'000 times on nodes. But if you take a closer look at these nodes, you will find typical patterns. I looked around Zürich and found (and fixed):
8 x leisure=park for mapping a single tree --> natural=tree (examples: node 3828157717, node 3927588215, node 3927588217, node 3927588213, node 3927588210, node 3927588211)
1 x leisure=park & name=Spielplatz --> create new area with leisure=playground (and without name)
8 x leisure=park & name=* inside an area (leisure=park or landuse=meadow) without name --> put tags on area

Special during #MapLesotho I found hundreds of single trees mapped with (simple to find with overpass)

• leisure=park
• leisure=park & natural=tree
• and my favourite: leisure=park & name=tree

If the node has no other tags, I only saw it as mapping for a tree. All I saw are created with ID. I try ID and found the following (misleading) behaviour:

• Start editing with ID.
• Set a new node.
• On the left side appears potential tags. The top key is park and the icon shows a tree.
• this adds a node with leisure=park
• Because these "trees" are not rendered, some mappers start to add additional tags like natural=tree or name=tree to get something shown on map.

We should at least warn about using leisure=park on a node.
We also could offer a fix for replacing the three most used cases (see above) with natural=tree

Last but not least ID shouldn’t offer leisure=park on nodes! (But instead natural=tree)