feat(poly): added skeleton of polytrope model

the polytrope module will be used as an initial guess to the solver. A skeleton of this has been imported from https://github.com/tboudreaux/FEMPolytrope

This module will need major updates still to handle 3D, proper boundary conditions, and to incorporate it with the rest of our meshing scheme

src/poly/coeff/private/polyCoeff.cpp

@@ -0,0 +1,18 @@
+#include "mfem.hpp"
+#include <cmath>
+
+#include "coeff.h"
+
+double xi_coeff_func(const mfem::Vector &x) {
+  return std::pow(x(0), 2);
+}
+
+void vec_xi_coeff_func(const mfem::Vector &x, mfem::Vector &v) {
+  v.SetSize(1);
+  v[0] = -std::pow(x(0), 2);
+}
+
+double theta_initial_guess(const mfem::Vector &x, double root) {
+  double xi = x[0];
+  return 1-std::pow(xi/root, 2);
+}