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/timestamp.c

@@ -13,8 +13,6 @@
 #include "bacnet/datetime.h"
 #include "bacnet/timestamp.h"
 
-/** @file timestamp.c  Encode/Decode BACnet Timestamps  */
-
 /** Set the sequence number in a timestamp structure.
  *
  * @param dest  Pointer to the destination time stamp structure.