Add first fully functioning M-callable C implementation of the

'ip_simple' mimetic inner product.  Coincides with the results from
  'computeMimeticIP' to (roughly) the order of round-off on a
  non-trivial test case.

  The calling interface is

      BI = mex_ip_simple(G, rock)

  and some latitude has been extended towards multiple data types in
  the various G fields (e.g., G.cells.facePos may be an 'int32').

  Additional clean-up and optimisation is likely.

mimetic.c

@@ -1,6 +1,12 @@
 #include <stddef.h>
 #include <stdlib.h>
 
+#if COMPILING_FOR_MATLAB
+#include "mex.h"
+#include "matrix.h"
+#define MAT_SIZE_T mwSignedIndex
+#endif
+
 #ifndef MAT_SIZE_T
 #define MAT_SIZE_T int
 #endif