feat(libmeanfield): variadic refactor

also added normaliztion operator

extension_example/demo.cpp

@@ -0,0 +1,43 @@
+#include <iomanip>
+#include <iostream>
+
+import mean_field;
+import mean_field_extension_example.rotating_stellar_model;
+
+int main() {
+    using namespace mean_field;
+    using namespace mean_field::extension_example;
+
+    const IdealGasRadiation equationOfState({
+        .meanMolecularWeight = 0.61,
+        .boltzmannConstant = 1.380649e-16,
+        .atomicMassUnit = 1.66053906660e-24,
+        .radiationConstant = 7.5657e-15
+    });
+
+    const dimensions::DensityValue density{10.0};       // g cm^-3
+    const dimensions::TemperatureValue temperature{1.5e7}; // K
+    const auto pressure = eos::evaluate<dimensions::quantity::Pressure>(
+        equationOfState,
+        density,
+        temperature
+    );
+
+    const auto model = makeRotatingStellarModel({
+        .equationOfState = equationOfState.parameters(),
+        .surfacePressure = dimensions::PressureValue{0.0},
+        .totalMass = dimensions::MassValue{1.0},
+        .totalAngularMomentum = dimensions::AngularMomentumValue{0.2}
+    });
+
+    std::cout << std::scientific
+              << "P(rho = 10 g cm^-3, T = 1.5e7 K) = "
+              << pressure.value() << " dyn cm^-2\n"
+              << "Compiled specification count = "
+              << model.specificationCount << '\n'
+              << "Current barotropic backend accepts this thermal model = "
+              << std::boolalpha
+              << currentEquilibriumBackendSupportsIdealGasRadiation << '\n';
+
+    return 0;
+}