fix(src): updated to compile on gcc and clang

gcc disallows some constexprs that work in clang, these have been removed to that SERiF compiles on gcc and clang

src/eos/public/EOS.h

@@ -148,12 +148,6 @@ namespace serif::eos {
          */
         double specificHeatCapacityAtConstantPressure();
 
-        /**
-         * @brief Returns the format of the EOS data used to generate this output.
-         * @return The EOSFormat enum value (currently only EOSFormat::HELM).
-         */
-        EOSFormat EOSFormat() const;
-
         friend std::ostream& operator<<(std::ostream& os, const EOSOutput& output) {
             os << "EOSOutput:\n"
                 << "\tElectron Fraction: " << output.electronFraction << "\n"