Add block_response helper method to process

Avoids Block needing to know about Process' internal BlockRespond
message.

malcolm/core/block.py

@@ -2,7 +2,6 @@
 from contextlib import contextmanager
 
 from malcolm.core.monitorable import Monitorable
-from malcolm.core.process import BlockRespond
 from malcolm.core.request import Request
 from malcolm.core.response import Response
 
@@ -72,7 +71,8 @@ def handle_request(self, request):
                 self._attributes[attr_name].put(request.value)
                 self._attributes[attr_name].set_value(request.value)
                 response = Response.Return(request.id_, request.context)
-            response = BlockRespond(response, request.response_queue)
+            response = self.parent.make_block_response_message(
+                response, request.response_queue)
             self.parent.q.put(response)
 
     def to_dict(self):

malcolm/core/process.py

@@ -203,6 +203,9 @@ def _handle_get(self, request):
         response = Response.Return(request.id_, request.context, d)
         request.response_queue.put(response)
 
+    def make_block_response_message(self, response, response_queue):
+        return BlockRespond(response, response_queue)
+
     def notify_subscribers(self, block_name):
         self.q.put(BlockNotify(name=block_name))