feat: add support for reading counters

Initiating and finishing a manual operation works a bit strange. First
of all the Maximum Manual Operation Time is controlled by a setting
(--id 4) and defaults to 0 (infinite). And instead of waiting for the
timer to expire (which you have to do when its set to 0) you can finish
a manual operation by passing "--no-check".

Note: For some reason no response is given when a manual operation is
written to. That's why execute_action() returns succesfully (with None)
when its executing the "setmanual" action and the response queue size is 0.

README.md

@@ -193,7 +193,7 @@ See the [pislave](https://github.com/ootjersb/pislave#wiring) project
   Are you really sure? (Type uppercase yes): YES
   ```
 
-* Reset all errors:
+* Reset timer:
   ```
   # ./itho-wpu.py --action setmanual --id 38 --value 1
   Current manual operation: