feat: add solver core skeleton

tests/unit/model_load_test.cpp

@@ -0,0 +1,11 @@
+#include <fesa/model/load.hpp>
+
+int main()
+{
+    const fesa::model::Load load{
+        fesa::core::NodeId{2},
+        fesa::model::DofComponent::ux,
+        5.0
+    };
+    return load.value() == 5.0 ? 0 : 1;
+}