LinkamBase: add missing methods _on_shutdown and initialize (issue #113)

* microscope/stages/linkam.py (LinkamBase): class was missing an
  implementation for the Device abstract methods _on_shutdown() and
  initialize().  This fixes the issue when creating a LinkCMS
  instance.  This was failing since 83f1256 (which fixed ABCs
  for Python 3).

Author: carandraug

microscope/stages/linkam.py

@@ -1018,6 +1018,12 @@ def __init__(self, **kwargs):
             self.init_sdk()
         self._reconnect_thread = None
 
+    def initialize(self):
+        pass
+
+    def _on_shutdown(self):
+        pass
+
     def __del__(self):
         """Close comms on object deletion"""
         self._process_msg(Msg.CloseComms)