Extra field for mechanism instance (to be compatible with next mod2c update) (#122)

Author: pramodk

coreneuron/nrnoc/nrnoc_ml.h

@@ -89,6 +89,7 @@ struct Memb_list {
     NetSendBuffer_t* _net_send_buffer;
     int nodecount; /* actual node count */
     int _nodecount_padded;
+    void* instance; /* mechanism instance */
 };
 }  // namespace coreneuron
 #endif