Fix sliproads scenario 4

[oxidase]

Issue

PR fixes scenario 4 of #4348 by allowing sliproads from links via links.

Tasklist

• add regression / cucumber cases (see docs/testing.md)
• make a map of locations with modified instructions
• review
• adjust for comments

Requirements / Relations

Link any requirements here. Other pull requests this PR is based on?

[daniel-j-h]

Removing the link-class check here looks good but we really need to look at real-world situations to make a call here.

• needs map to inspect situations we now classify as sliproads
• needs update to our maneuver classification paper

[MoKob]

@oxidase before reviewing, could you provide the map of real-world situations that @daniel-j-h asked for. Without that, the implications of your changes are impossible to assess.
With that map, look at approximately hundred changes and make a note of improved / worsened classification (jugdement call: is this a single turn or is it two).

[oxidase]

@MoKob here a map in DE and CA with 671 modified locations https://api.mapbox.com/styles/v1/oxidase/cj8ctmb3z8il92qmnnpzkt3q2.html?fresh=true&title=true&access_token=pk.eyJ1Ijoib3hpZGFzZSIsImEiOiJjaXcxNWMxYmswMDRlMnlxOWtlem0yamU3In0.PjtHAtgQDzxhZ1BE484Sqw#7.19/51.554/6.938

Most of nodes are like https://www.mapbox.com/get-directions/#17.24/51.54773/7.21275?coordinates=7.211749965246469,51.54783232143697;7.212753812634247,51.54772639593111
or https://www.mapbox.com/get-directions/#15.68/51.3893/6.2664?coordinates=6.267416425333607,51.387693856479984;6.267469002825692,51.388199698191016 with two instructions merged to a single instruction.

But in some rare cases https://www.mapbox.com/get-directions/#16.75/37.86651/-122.30449?coordinates=-122.30407045878566,37.86592529445568;-122.30339574159481,37.86677712776516 where a single instruction changed to "Turn right onto University Avenue"

[MoKob]

The result of the changes I've seen on the map look fine. I am questioning whether allowing links in general as main road could be a bit too generous, though. This PR is mostly targeting ramps, right? Do you think we could achieve the same with just allowing motorway-links, but not all links?

[MoKob]

*shortcut

[MoKob]

Have you tried what happens, if you explicitly allow MotorwayLink classes here? Ramps are different than the primary/.. links, right?

[oxidase]

@MoKob I have updated the map with a condition first_road_data.road_classification.IsLinkClass() && !first_road_data.road_classification.IsRampClass() to use only ramp-> link -> non-link (red nodes) instead of any -> link -> non-link and number of affected intersections halved
https://api.mapbox.com/styles/v1/oxidase/cj8ctmb3z8il92qmnnpzkt3q2.html?title=true&access_token=pk.eyJ1Ijoib3hpZGFzZSIsImEiOiJjaXcxNWMxYmswMDRlMnlxOWtlem0yamU3In0.PjtHAtgQDzxhZ1BE484Sqw#18.27/49.46387/8.50602

A typical example is https://www.mapbox.com/get-directions/#18.27/49.46407/8.50528?coordinates=8.50620676926198,49.464227828009285;8.505281415385014,49.46406952432184
with a primary link -> primary link -> primary sliproad

[MoKob]

@oxidase in that case, we can keep it with the complete removal. It looks like we didn't regress anywhere (or at least in no location I've seen). Good to go from my point of view.