Kommon: enable build flag

Kommon/Core/Utility/KConst.h

@@ -2,8 +2,15 @@
  * @file KConst.h
  * @author W. Kaefer
  * @author M. Kleesiek <marco.kleesiek@kit.edu>
+ * @author J. Behrens <jan.behrens@kit.edu>
+ * @author S. Hickford <stephanie.hickford@kit.edu>
  */
 
+#if KConst_REFERENCE_EPOCH == 2006
+#include "KConst_2006.h"
+
+#elif KConst_REFERENCE_EPOCH == 2021
+
 #ifndef KCONST_H_
 #define KCONST_H_
 
@@ -403,7 +410,7 @@ constexpr double Viscosity_T2_30K()
 constexpr double TemperatureSlipCoefficient()
 {
     return 1.175;
-}   //!< temperature slip coefficient, unit: none, Ref: No easy interpolation possible, from Sharipov, Tab 4, S-model "Data on the velocity slip and temperature jump coefficients [gas, mass, heat, and momentum transfer] https://doi.org/10.1109/ESIME.2004.1304046 
+}   //!< temperature slip coefficient, unit: none, Ref: No easy interpolation possible, from Sharipov, Tab 4, S-model "Data on the velocity slip and temperature jump coefficients [gas, mass, heat, and momentum transfer] https://doi.org/10.1109/ESIME.2004.1304046
 
 constexpr double M_Si()
 {
@@ -444,3 +451,7 @@ constexpr double Ue3sq()
 } /* namespace katrin */
 
 #endif  //KCONST_H
+
+#else
+#error "Unsupported value for KConst_REFERENCE_EPOCH."
+#endif