feat: add domain model foundation

tests/core/load_definition_test.cpp

@@ -0,0 +1,25 @@
+#include "fesa/core/LoadDefinition.hpp"
+
+namespace {
+
+int require(bool condition) {
+    return condition ? 0 : 1;
+}
+
+} // namespace
+
+int run_load_definition_tests() {
+    const fesa::core::NodalLoadDefinition load{1, fesa::core::Dof::U3, -100.0};
+
+    if (const int result = require(load.nodeId() == 1); result != 0) {
+        return result;
+    }
+    if (const int result = require(load.dof() == fesa::core::Dof::U3); result != 0) {
+        return result;
+    }
+    if (const int result = require(load.value() == -100.0); result != 0) {
+        return result;
+    }
+
+    return 0;
+}