piste:type validation improvement

[yvecai]

While piste:difficulty is more than just useful for the data user, piste:grooming for crosscountry skiing appears to be defaults to quite different practises across the world.

--- combinations.mapcss	2019-04-14 08:10:57.452104953 +0200
+++ mods/combinations.mapcss	2019-04-14 08:17:18.765109192 +0200
@@ -119,6 +119,8 @@
 /* {0.tag} without {1.key} (warning level) */
 node[emergency=fire_hydrant][!fire_hydrant:type],
 way[     boundary=administrative][!admin_level],
+way[piste:type=nordic][!piste:grooming][!piste:difficulty],
+way[piste:type=downhill][!piste:difficulty],
 relation[boundary=administrative][!admin_level],
 relation[route=bicycle ][!network][type=route],
 relation[route=hiking  ][!network][type=route],

[yvecai]

patch file