code tidy: inline karte_t (k) functions now inline x,y variants, removed duplicate function planquadrat_t *lookup(const koord &k) - access(k) does same job

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6620 8aca7d54-2c30-db11-9de9-000461428c89