Add test for mutant

src/tests/catch_test_mutant.cxx

+#include "catch/catch.hpp"
+
+#include "teqp/models/multifluid.hpp"
+
+TEST_CASE("Test construction of mutant", "[mutant]")
+{
+
+	std::string coolprop_root = "../mycp";
+	auto BIPcollection = coolprop_root + "/dev/mixtures/mixture_binary_pairs.json";
+
+	auto model = build_multifluid_model({ "Nitrogen", "Ethane" }, coolprop_root, BIPcollection);
+
+    std::string s0 = R"({"0": {} })";
+    nlohmann::json j0 = nlohmann::json::parse(s0);
+
+    std::string s = R"({
+        "0": {
+            "1": {
+                "BIP": {
+                    "betaT": 1.1,
+                    "gammaT": 0.9,
+                    "betaV": 1.05,
+                    "gammaV": 1.3,
+                    "Fij": 1.0
+                },
+                "departure":{
+                    "type": "none"
+                }
+            }
+        }
+    })";
+	nlohmann::json j = nlohmann::json::parse(s);
+    auto mutant = build_multifluid_mutant(model, j);
+
+    double T = 300, rho = 300;
+    Eigen::ArrayXd molefrac(2); molefrac = 0.5;
+    auto Ar02base = TDXDerivatives<decltype(model)>::get_Ar02(model, T, rho, molefrac);
+    auto Ar02mut = TDXDerivatives<decltype(mutant)>::get_Ar02(mutant, T, rho, molefrac);
+    CHECK(Ar02base != Ar02mut);
+
+}
\ No newline at end of file