Add qmckl_extra pointer

org/qmckl_context.org

@@ -141,8 +141,18 @@ typedef struct qmckl_context_struct {
   /* To be implemented:
   ,*/
 
+ /* Pointer to implementation-specific data */
+
+  void* qmckl_extra;
+
 } qmckl_context_struct;
    #+end_src
+    
+   The qmckl_extra pointer lets the other implementation of the library
+   add specific things to the context. For example a GPU implementation
+   of QMCkl will need to store the device ID in the context, and this
+   can be made by creating a private data structure containing all 
+   GPU-specific data, including the device ID.
 
    A tag is used internally to check if the memory domain pointed
    by a pointer is a valid context. This allows to check that even if