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/CMakeLists.txt

@@ -116,6 +116,7 @@ list(APPEND testdirs
   bacnet/rpm
   bacnet/secure_connect
   bacnet/specialevent
+  bacnet/timer_value
   bacnet/timestamp
   bacnet/timesync
   bacnet/weeklyschedule
@@ -177,6 +178,7 @@ list(APPEND testdirs
   bacnet/basic/object/schedule
   bacnet/basic/object/structured_view
   bacnet/basic/object/time_value
+  bacnet/basic/object/timer
   bacnet/basic/object/trendlog
   # basic/program
   bacnet/basic/program/ubasic