feat(poly): interpolating polynomial to find polytrope surface

Instead of treating the polytrope as a free boundary problem I have defined an interpolating polynominal, accurate to within 0.01 percent over n=[0,5) which is used to set the size of the domain for a given n

src/poly/coeff/private/polyCoeff.cpp

@@ -19,7 +19,7 @@
 //
 // *********************************************************************** */
 #include "mfem.hpp"
-#include <cmath>
+#include <iostream>
 
 #include "polyCoeff.h"
 
@@ -35,4 +35,14 @@ namespace polycoeff{
     v.SetSize(3);
     for (int i = 0; i < 3; i++) { v(i) = -1; }
   }
+
+  double x1(const double n)
+  {
+    double r = 0;
+    for (int i = 0; i < x1InterpCoeff::numTerms; i++) {
+      r += x1InterpCoeff::coeff[i] * std::pow(n, x1InterpCoeff::power[i]);
+    }
+    return r;
+  }
+
 }