Add default grid constructor. Creates an empty grid.

This is mostly for use in *other* grid constructors, as it enables
putting a grid into a defined state before allocating and/or filling
the specific fields of the UnstructuredGrid.

While here, add a Doxygen \file command lest function documentation
not be output to the documentation set.

opm/core/grid.c

@@ -44,3 +44,17 @@ void destroy_grid(struct UnstructuredGrid *g)
     free(g);
 }
 
+
+struct UnstructuredGrid *
+create_grid_empty(void)
+{
+    struct UnstructuredGrid *G, g = { 0 };
+
+    G = malloc(1 * sizeof *G);
+
+    if (G != NULL) {
+        *G = g;
+    }
+
+    return G;
+}

opm/core/grid.h

@@ -20,6 +20,12 @@
 #ifndef OPM_GRID_HEADER_INCLUDED
 #define OPM_GRID_HEADER_INCLUDED
 
+/**
+ * \file
+ *
+ * Main OPM-Core grid data structure along with destructor and default
+ * constructor.
+ */
 
 #ifdef __cplusplus
 extern "C" {
@@ -221,6 +227,17 @@ struct UnstructuredGrid
  */
 void destroy_grid(struct UnstructuredGrid *g);
 
+/**
+   Allocate and initialise an empty UnstructuredGrid.
+
+   This is the moral equivalent of a C++ default constructor.
+
+   \return Fully formed UnstructuredGrid with all fields zero or
+   <code>NULL</code> as appropriate.  <code>NULL</code> in case of
+   allocation failure.
+ */
+struct UnstructuredGrid *
+create_grid_empty(void);
 
 #ifdef __cplusplus
 }