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.

test/bacnet/datalink/bsc-datalink/CMakeLists.txt

@@ -195,6 +195,7 @@ target_sources(${PROJECT_NAME} PRIVATE
   ${SRC_DIR}/bacnet/special_event.c
   ${SRC_DIR}/bacnet/channel_value.c
   ${SRC_DIR}/bacnet/secure_connect.c
+  ${SRC_DIR}/bacnet/timer_value.c
   ${SRC_DIR}/bacnet/timestamp.c
   ${SRC_DIR}/bacnet/weeklyschedule.c
   ${SRC_DIR}/bacnet/wp.c