UnitSystem: Make DefaultCulture settable

Except in WinRTC, which does not support IFormatProvider type.

Author: angularsen

UnitsNet/CustomCode/UnitSystem.cs

@@ -137,8 +137,15 @@ public UnitSystem() : this(DefaultCulture)
 
         /// <summary>
         ///     Defaults to <see cref="CultureInfo.CurrentUICulture" /> when creating an instance with no culture provided.
+        ///     Can be overridden, but note that this is static and will affect all subsequent usages.
         /// </summary>
-        private static IFormatProvider DefaultCulture => CultureInfo.CurrentUICulture;
+#if WINDOWS_UWP
+        // Windows Runtime Component does not support exposing the IFormatProvider type in public API
+        internal
+#else
+        public
+#endif
+            static IFormatProvider DefaultCulture { get; set; } = CultureInfo.CurrentUICulture;
 
         public bool IsFallbackCulture => Culture.Equals(FallbackCulture);