Use a sensible semi-placeholder soundspeed

src/equation_of_state/planetary/tillotson.h

@@ -45,7 +45,6 @@
 struct Til_params {
     int mat_id;
     float rho_0, a, b, A, B, E_0, E_iv, E_cv, alpha, beta, eta_min, P_min;
-    float c_TEMPORARY;
 };
 
 // Parameter values for each material (cgs units)
@@ -63,8 +62,6 @@ INLINE static void set_Til_iron(struct Til_params *mat, int mat_id) {
     mat->beta = 5.0;
     mat->eta_min = 0.0;
     mat->P_min = 0.0;
-
-    mat->c_TEMPORARY = 9.4e-4;
 }
 INLINE static void set_Til_granite(struct Til_params *mat, int mat_id) {
     mat->mat_id = mat_id;
@@ -80,8 +77,6 @@ INLINE static void set_Til_granite(struct Til_params *mat, int mat_id) {
     mat->beta = 5.0;
     mat->eta_min = 0.0;
     mat->P_min = 0.0;
-
-    mat->c_TEMPORARY = 9.4e-4;
 }
 INLINE static void set_Til_water(struct Til_params *mat, int mat_id) {
     mat->mat_id = mat_id;
@@ -97,8 +92,6 @@ INLINE static void set_Til_water(struct Til_params *mat, int mat_id) {
     mat->beta = 5.0;
     mat->eta_min = 0.915;
     mat->P_min = 0.0;
-
-    mat->c_TEMPORARY = 9.4e-4;
 }
 
 // Convert from cgs to internal units
@@ -112,8 +105,6 @@ INLINE static void convert_units_Til(
     mat->E_iv /= units_cgs_conversion_factor(us, UNIT_CONV_ENERGY_PER_UNIT_MASS);
     mat->E_cv /= units_cgs_conversion_factor(us, UNIT_CONV_ENERGY_PER_UNIT_MASS);
     mat->P_min /= units_cgs_conversion_factor(us, UNIT_CONV_PRESSURE);
-
-    mat->c_TEMPORARY /= units_cgs_conversion_factor(us, UNIT_CONV_SPEED);
 }
 
 // gas_internal_energy_from_entropy
@@ -276,7 +267,7 @@ INLINE static float Til_soundspeed_from_internal_energy(
 //    }
     float c;
 
-    c = mat->c_TEMPORARY; /// VERY TEMPORARY!!!
+    c = sqrt(mat->A / mat->rho_0);
 
     return c;
 }
@@ -287,7 +278,7 @@ INLINE static float Til_soundspeed_from_pressure(
 
     float c;
 
-    c = mat->c_TEMPORARY; /// VERY TEMPORARY!!!
+    c = sqrt(mat->A / mat->rho_0);
 
     return c;
 }