feat(poly): refactoring PolytropeOperator to work on the reduced system so as to avoid rank deficiencies

src/poly/utils/public/utilities.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include "mfem.hpp"
+
+namespace serif::utilities {
+    mfem::SparseMatrix buildReducedMatrix(
+        const mfem::SparseMatrix& matrix,
+        const mfem::Array<int>& trialEssentialDofs,
+        const mfem::Array<int>& testEssentialDofs
+    );
+}