change enum OpacityModel to enum class OpacityModel

src/radiation_system.hpp

@@ -52,7 +52,7 @@ static constexpr double inf = std::numeric_limits<double>::max();
 static const bool use_wavespeed_correction = false;
 
 // enum for opacity_model
-enum OpacityModel {
+enum class OpacityModel {
 	user = 0,	  // user-defined opacity for each group, given as a function of density and temperature.
 	piecewisePowerLaw // piecewise power-law opacity model with piecewise power-law fitting to a user-defined opacity function and on-the-fly piecewise
 			  // power-law fitting to radiation energy density and flux.