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/public/polyCoeff.h

@@ -0,0 +1,8 @@
+#include "mfem.hpp"
+#include <cmath>
+
+double xi_coeff_func(const mfem::Vector &x);
+
+void vec_xi_coeff_func(const mfem::Vector &x, mfem::Vector &v);
+
+double theta_initial_guess(const mfem::Vector &x, double root);