Fix issue 2656

This is a fix for issue #2656.

Search for a locator named `tag_landing` which is a point located at the lowest y-coordinate position of a model that the feet reach down too.

Previously we used the bounding box but this is no longer valid behaviour since the shield mesh can extend this some distance past the limits that the landing feet/wheels can reach.

NB: @nozmajner I haven't added the model you gave me for testing at the time I'm opening this PR.