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/bacapp.h

@@ -30,6 +30,7 @@
 #include "bacnet/calendar_entry.h"
 #include "bacnet/special_event.h"
 #include "bacnet/channel_value.h"
+#include "bacnet/timer_value.h"
 #include "bacnet/secure_connect.h"
 
 #ifndef BACAPP_PRINT_ENABLED
@@ -67,7 +68,12 @@ typedef struct BACnet_Application_Data_Value {
     uint8_t context_tag; /* only used for context specific data */
     uint8_t tag; /* application tag data type */
     union {
-        /* NULL - not needed as it is encoded in the tag alone */
+#if defined(BACAPP_NULL)
+        /* no value needed because it is encoded in the tag alone */
+#endif
+#if defined(BACAPP_NO_VALUE)
+        /* no value needed because it is encoded in the tag alone */
+#endif
 #if defined(BACAPP_BOOLEAN)
         bool Boolean;
 #endif
@@ -170,6 +176,9 @@ typedef struct BACnet_Application_Data_Value {
 #if defined(BACAPP_CHANNEL_VALUE)
         BACNET_CHANNEL_VALUE Channel_Value;
 #endif
+#if defined(BACAPP_TIMER_VALUE)
+        BACNET_TIMER_STATE_CHANGE_VALUE Timer_Value;
+#endif
 #if defined(BACAPP_LOG_RECORD)
         BACNET_LOG_RECORD Log_Record;
 #endif