Add missing pragma once on headers

Implicit.hpp

@@ -1,3 +1,5 @@
+#pragma once
+
 class ImplicitSurface {
     public:
         virtual double operator() (double x, double y, double z);

Mesh.hpp

@@ -2,6 +2,8 @@
  * Defines a mesh, ready to be OpenGL-rendered
  **/
 
+#pragma once
+
 #include <vector>
 #include <unordered_map>
 #include <cstddef>  // size_t

common_structures.hpp

@@ -2,6 +2,8 @@
  * Defines a few widely used, widely spread structures. Imported pervasively.
  **/
 
+#pragma once
+
 #include <functional>  // Hash
 #include <vector>
 #include <cassert>

render/GlutRender.hpp

@@ -1,5 +1,7 @@
 /** The most basic renderer — a stupid glut application */
 
+#pragma once
+
 #include "../Mesh.hpp"
 #include <set>
 

util/ObjParser.hpp

@@ -1,5 +1,7 @@
 /** Parses .obj mesh files, outputting a `Mesh` */
 
+#pragma once
+
 #include <fstream>
 #include <string>
 #include <stdexcept>