Block difficult mtb_rating segments for foot, see discussion in graph…

…hopper#3050 (graphhopper#3073)

* Block difficult mtb_rating segments for foot, see discussion in graphhopper#3050

* Improve documentation for mtb_rating and horse_rating

* Changes triggered fram review comments

core/src/main/resources/com/graphhopper/custom_models/foot.json

@@ -1,13 +1,14 @@
 // to use this custom model you need to set the following option in the config.yml
 // graph.elevation.provider: srtm   # enables elevation
-// graph.encoded_values: foot_access, hike_rating, foot_priority, foot_average_speed, average_slope
+// graph.encoded_values: foot_access, hike_rating, mtb_rating, foot_priority, foot_average_speed, average_slope
 // profiles:
 //    - name: foot
 //      custom_model_files: [foot.json, foot_elevation.json]
 
 {
   "priority": [
     { "if": "!foot_access || hike_rating >= 2", "multiply_by": "0" },
+    { "if": "mtb_rating > 2",  "multiply_by": "0" },
     { "else": "", "multiply_by": "foot_priority"}
   ],
   "speed": [

docs/core/custom-models.md

@@ -98,7 +98,9 @@ There are also some that take on a numeric value, like:
 
 - average_slope: a number for 100 * "elevation change" / edge_distance for a road segment; it changes the sign in reverse direction; see max_slope
 - curvature: "beeline distance" / edge_distance (0..1) e.g. a curvy road is smaller than 1
-- hike_rating, horse_rating, mtb_rating: a number from 0 to 6 for the `sac_scale` in OSM, e.g. 0 means "missing", 1 means "hiking", 2 means "mountain_hiking" and so on
+- hike_rating: a number from 0 to 6 for the `sac_scale` in OSM, e.g. 0 means "missing", 1 means "hiking", 2 means "mountain_hiking", 3 means demanding_mountain_hiking, 4 means alpine_hiking, 5 means demanding_alpine_hiking, and 5 means difficult_alpine_hiking
+- mtb_rating: a number from 0 to 7 for the `mtb:scale` in OSM, e.g. 0 means "missing", 1 means `mtb:scale=0`, 2 means `mtb:scale=1` and so on. A leading "+" or "-" character is ignored.  
+- horse_rating: a number from 0 to 6 for the `horse_scale` in OSM, e.g. 0 means "missing", 1 means "common", 2 means "demanding", 3 means difficult, 4 means critical, 5 means dangerous, and 6 means impossible
 - lanes: number of lanes
 - max_slope: a signed decimal for the maximum slope (100 * "elevation change / distance_i") of an edge with `sum(distance_i)=edge_distance`. Important for longer road segments where ups (or downs) can be much bigger than the average_slope.
 - max_speed: the speed limit from a sign (km/h)