Boats shouldn't be able to cross isthmuses

[QuintillusCFC]

If you have water tiles at N/S and land tiles at E/W, that's an isthmus and sea units can't move between the water tiles. But if N is a city, sea units can move in and out of it.

Currently, the AI uses the GetCoastNeighbors method in Tile.cs to determine if it can move into a water tile. This doesn't check for isthmuses. It should.

This restriction should also apply to human-controlled navies.

[KulkoBSW]

I am working on that ticket.