Added basic timer object, internal state machine, and unit tests (#1123)

* Added basic timer object, internal state machine, and unit tests

* Added BACnetTimerStateChangeValue encode, decode, parse, print, and diff with unit tests

* Changed handler of add/remove list element to check if the property is a BACnetLIST

* Added BACnetLIST utility for handling WriteProperty to a list.

* Fixed outlier ReadProperty object handlers to return zero when the RP parameter is NULL.

src/bacnet/basic/object/client/device-client.c

@@ -1216,6 +1216,9 @@ int Device_Read_Property(BACNET_READ_PROPERTY_DATA *rpdata)
     struct special_property_list_t property_list;
 #endif
 
+    if (!rpdata) {
+        return 0;
+    }
     /* initialize the default return values */
     rpdata->error_class = ERROR_CLASS_OBJECT;
     rpdata->error_code = ERROR_CODE_UNKNOWN_OBJECT;