Intersections from surfaces with two solutions are not updated correctly

If a surface intersection returns two solutions and they are written to the navigator cache, the ```intersection_update``` kernel has no mechanism by which to distinguish the near from the far solution, so at the moment it updates both candidates with the near solution (```sfi[0]```). This doubles that solution in the cache and removes the second solution, which will then never be found by the navigator.

Not an immediate concern, as I don't think this case is part of the current setups.