Thermodynamic Investigation of the Eutectic Mixture of the $$\hbox {LiNO}_{3}$$LiNO3–$$\hbox {NaNO}_{3}$$NaNO3–$$\hbox {KNO}_{3}$$KNO3–$$\hbox {Ca}(\hbox {NO}_{3})_{2}$$Ca(NO3)2 System