DS402: Cache supported operation modes (#247)

* ds402: Cache supported operation modes.

Requesting the same immutable OD object on each change of operation
mode takes quite some time to process.  Introduce a local cache member
variable in order to avoid requesting more than once.

* ds402: Log message about cache operation mode support.

canopen/profiles/p402.py

@@ -388,8 +388,13 @@ def is_op_mode_supported(self, mode):
         :return: If the operation mode is supported
         :rtype: bool
         """
-        mode_support = self.sdo[0x6502].raw & OperationMode.SUPPORTED[mode]
-        return mode_support == OperationMode.SUPPORTED[mode]
+        if not hasattr(self, '_op_mode_support'):
+            # Cache value only on first lookup, this object should never change.
+            self._op_mode_support = self.sdo[0x6502].raw
+            logger.info('Caching node {n} supported operation modes 0x{m:04X}'.format(
+                n=self.id, m=self._op_mode_support))
+        bits = OperationMode.SUPPORTED[mode]
+        return self._op_mode_support & bits == bits
 
     def on_TPDOs_update_callback(self, mapobject):
         """This function receives a map object.