make physical constants writeable

VectoCore/Utils/Physics.cs

@@ -42,12 +42,12 @@ namespace TUGraz.VectoCore.Utils
 		/// <summary>
 		/// Density of air.
 		/// </summary>
-		public static readonly SI AirDensity = 1.188.SI().Kilo.Gramm.Per.Cubic.Meter;
+		public static SI AirDensity = 1.188.SI().Kilo.Gramm.Per.Cubic.Meter;
 
 		/// <summary>
 		/// Density of fuel.
 		/// </summary>
-		public static readonly SI FuelDensity = 0.832.SI().Kilo.Gramm.Per.Cubic.Dezi.Meter;
+		public static SI FuelDensity = 0.832.SI().Kilo.Gramm.Per.Cubic.Dezi.Meter;
 
 		public static readonly double RollResistanceExponent = 0.9;
 
@@ -57,6 +57,6 @@ namespace TUGraz.VectoCore.Utils
 		/// <summary>
 		/// fuel[kg] => co2[kg]. Factor to convert from fuel weight to co2 weight.
 		/// </summary>
-		public static readonly double CO2PerFuelWeight = 3.16;
+		public static double CO2PerFuelWeight = 3.16;
 	}
 }
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