RF: refactor import_array as documented

Instead of calling `import_array()` in every C function, use the method
of sharing the numpy API pointer table documented here:

http://docs.scipy.org/doc/numpy/reference/c-api.array.html#importing-the-api

Also, deal with different return types from `import_array()` in Python 3
and Python 2 - see
http://mail.scipy.org/pipermail/numpy-discussion/2010-December/054345.html

lib/fff_python_wrapper/fffpy.c

@@ -9,12 +9,14 @@
    because PyArray_API is defined static, in order not to share that symbol
    within the dso. (import_array() asks the pointer value to the python process)
 */
-void fffpy_import_array(void) { 
-  import_array(); 
-  return;
+/*
+ * deal with differences in macro return result between Python 2 and 3
+ * http://mail.scipy.org/pipermail/numpy-discussion/2010-December/054350.html
+ */
+IMP_OUT fffpy_import_array(void) {
+  import_array();
 }
 
-
 /* Static functions */
 static npy_intp _PyArray_main_axis(const PyArrayObject* x, int* ok); 
 static fff_vector* _fff_vector_new_from_buffer(const char* data, npy_intp dim, npy_intp stride, int type, int itemsize);

lib/fff_python_wrapper/fffpy.h

@@ -31,8 +31,17 @@
    static, in order not to share that symbol within the
    dso. (import_array() asks the pointer value to the python process)
 */
-extern void fffpy_import_array(void);
+/*
+ * deal with differences in macro return result between Python 2 and 3
+ * http://mail.scipy.org/pipermail/numpy-discussion/2010-December/054350.html
+ */
+#if PY_MAJOR_VERSION >= 3
+typedef int IMP_OUT;
+#else
+typedef void IMP_OUT;
+#endif
 
+extern IMP_OUT fffpy_import_array(void);
 
 /*!
   \brief Convert \c PyArrayObject to \c fff_vector