Replace get/set functions with properties

#1421 

Replace get/set functions in:

ArcGisMapServerImageryProvider, BingMapsImageryProvider, GoogleEarthImateryProvider, GridImageryProvider, OpenStreetMapImageryProvider, SingleTileImageryProvider, TileCoordinatesImageryProvider, TileMapServiceImageryProvider, WebMapServiceImageryProvider

CesiumTerrainProvider, EllipsoidTerrainProvider, ArcGisImageServerTerrainProvider, VRTheWorldTerrainProvider