feat(polyInitialGuess): use numeric derivitive for phi initial guess

src/poly/solver/private/polySolver.cpp

@@ -270,16 +270,8 @@ void PolySolver::setInitialGuess() const {
             return -std::pow((u*r), 2)+1.0;
         }
     );
-    mfem::VectorFunctionCoefficient phiInitGuess (m_mesh->SpaceDimension(),
-        [this](const mfem::Vector &x, mfem::Vector &v) {
-            double radius = Probe::getMeshRadius(*m_mesh);
-            double u = -1/std::pow(radius,2);
-            v(0) = 2*x(0)*u;
-            v(1) = 2*x(1)*u;
-            v(2) = 2*x(2)*u;
-        }
-    );
     m_theta->ProjectCoefficient(thetaInitGuess);
+    mfem::GradientGridFunctionCoefficient phiInitGuess (m_theta.get());
     m_phi->ProjectCoefficient(phiInitGuess);
     if (m_config.get<bool>("Poly:Solver:ViewInitialGuess", false)) {
         Probe::glVisView(*m_theta, *m_mesh, "θ init");