Check upgrade path preservation

[Auzzy]

In the board state CSVs, 18xx-routes supports upgrading tiles. That is, if multiple tiles are present with the same coordinate, as long as they have different upgrade levels, it's accepted, and the newer tile is placed.
However, no check is made to confirm the paths of the older tile were preserved. The only reason I can imagine a user using this upgrade path is to keep a running log of tiles placed. So it's natural to assume the older paths should be preserved.