Be more aggresive classifying Roundabout Intersections.

Roundabout Intersections are roundabouts with up to four ways and turn
angles which makes the turns obvious, e.g. as in:

```
    *
    *
* *   * *
    *
    *
```

but not

```
     *
    *
* *   *
    * *
     * *
```

For Roundabout Intersections we issue instructions such as
"turn <direction>" instead of "take the <nth> exit".

At the moment we have a limit on the radius for these Roundabout
Intersections of 5 meters. Which fails to classify a wide range of
Roundabout Intersections in the US (with the US-wide streets).

This changeset removes the Roundabout Intersection radius limit:

- if the roundabout is larger than a threshold and is named we classify
  it as a rotary

- if the roundabout matches our criteria for Roundabout Intersections
  we classify it as a Roundabout Intersection

- else fallback to plain old Roundabout

There is a second issue with determining a roundabout's radius.
But that's for another pull request (tracking in #2716).

References:
- https://github.com/Project-OSRM/osrm-backend/issues/2716

include/extractor/guidance/constants.hpp

@@ -21,8 +21,12 @@ const double constexpr GROUP_ANGLE = 60;
 const double constexpr FUZZY_ANGLE_DIFFERENCE = 25.;
 const double constexpr DISTINCTION_RATIO = 2;
 
-const double constexpr MAX_ROUNDABOUT_INTERSECTION_RADIUS = 5;
-const double constexpr MAX_ROUNDABOUT_RADIUS = 15; // 30 m diameter as final distinction
+// Named roundabouts with radii larger then than this are seen as rotary
+const double constexpr MAX_ROUNDABOUT_RADIUS = 15;
+// Unnamed small roundabouts that look like intersections are announced as turns,
+// guard against data issues or such roundabout intersections getting too large.
+const double constexpr MAX_ROUNDABOUT_INTERSECTION_RADIUS = 25;
+
 const double constexpr INCREASES_BY_FOURTY_PERCENT = 1.4;
 
 const int constexpr MAX_SLIPROAD_THRESHOLD = 250;