Tile improvement fix

[stavrosfa]

I had some issues when trying to load a json save where some tiles had railroads. As far as I can tell, this happens, because when we save the overlays, if there is a railroad, we don't save the road that invisibly exists underneath it.

So then, when loading, we try to add railroad to a tile that doesn't seem to have a road.

This does not happen with .biq and .sav files because when debugging I saw that we import both a road and a railroad overlay when laoding the tile info

So now, the only place where we check if we can add an improvement, is during gameplay and not when loading a save. I think it also makes sense from a cleaner code standpoint, to have a dedicated method to add an improvement, rather than also checking if we can add it in the same method.

I don't think that we need to imitate what the .biq and .sav files do and save both improvements, as we can upgrade and downgrade on the fly.

[ajhalme]

Seems reasonable.