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/basic/object/av/CMakeLists.txt

@@ -54,6 +54,7 @@ add_executable(${PROJECT_NAME}
     ${SRC_DIR}/bacnet/hostnport.c
     ${SRC_DIR}/bacnet/lighting.c
     ${SRC_DIR}/bacnet/proplist.c
+    ${SRC_DIR}/bacnet/timer_value.c
     ${SRC_DIR}/bacnet/timestamp.c
     ${SRC_DIR}/bacnet/memcopy.c
     ${SRC_DIR}/bacnet/wp.c