Make geod a let constant.

Author: scottrhoyt

Sources/GeographicLib/Geodesic.swift

@@ -16,7 +16,7 @@ import CGeographicLib
 ///
 /// The default ellipsoid is WGS-84, but custom ellipsoids can be specified.
 public struct Geodesic: Sendable {
-    internal private(set) var geod: geod_geodesic
+    internal let geod: geod_geodesic
 
     // MARK: - Initialization
 
@@ -30,8 +30,9 @@ public struct Geodesic: Sendable {
     ///   - equatorialRadius: The equatorial radius in meters
     ///   - flattening: The flattening of the ellipsoid
     public init(equatorialRadius: Double, flattening: Double) {
-        self.geod = geod_geodesic()
-        geod_init(&self.geod, equatorialRadius, flattening)
+        var geod = geod_geodesic()
+        geod_init(&geod, equatorialRadius, flattening)
+        self .geod = geod
     }
 
     // MARK: - Properties