Error if both cv and mu are set. Add cv and mu to params

src/gas/gas.cpp

@@ -111,11 +111,18 @@ std::shared_ptr<StateDescriptor> Initialize(ParameterInput *pin,
       cv = pin->GetReal("gas", "cv");
       PARTHENON_REQUIRE(cv > 0, "Only positive cv allowed!");
       mu = constants.GetKBCode() / ((gamma - 1.) * constants.GetAMUCode() * cv);
+      if (pin->DoesParameterExist("gas", "mu")) {
+        PARTHENON_FAIL("Cannot set both mu and cv in the input file. One is calculated "
+                       "from the other!");
+      }
     } else {
       mu = pin->GetOrAddReal("gas", "mu", 1.);
       PARTHENON_REQUIRE(mu > 0, "Only positive mean molecular weight allowed!");
       cv = constants.GetKBCode() / ((gamma - 1.) * constants.GetAMUCode() * mu);
     }
+    // Store these so they are available in outputs
+    params.Add("cv", cv);
+    params.Add("mu", mu);
     auto zbar = pin->GetOrAddReal("gas", "zbar", 1.0);
     EOS eos_host = singularity::IdealGas(gamma - 1., cv, {mu, zbar});
     EOS eos_device = eos_host.GetOnDevice();