Adding trade routes

[miniature-tiger]

• Trade route SVG board layer added (createTradeRouteLayer)
• Method (tradeRoute) added to draw an SVG path in team colours based on an array of tile coordinates
• Circle end points added for each end of SVG trade route path
• Method (initialiseTradePath) to initialise tradePath array which holds board size (i rows x j columns) array of paths
• Method (nearestHarbour) added which finds best sea harbour tiles next to Resource and Fort start and end points.
• Methods (discoverPath, pathTiles, pathCost) added which find most direct trade route between two points, avoiding islands.
• Trade route creation added to game flow logic.