Function tile_xy_to_geo 

pub fn tile_xy_to_geo(zoom: u8, x: f64, y: f64) -> GeoCoord

Convert fractional tile coordinates to geographic.

Accepts fractional (x, y) values that may sit on or beyond the tile grid boundary (for example x = n to get the east edge of the last column). This is useful for deriving geographic tile corners before reprojecting them into other planar world spaces.