tips for BA node

• behaviour analysis（short as BA） works dependently on tracking, all BA nodes Must attached after track node in pipeline.
• all types of BA nodes support multi-channel, single instance of BA node supports multi channels as input.
• all BA nodes can be divided into 2 categories, one is instantaneous(like crossline) and another one is continuous(like stop, jam). vp_ba_type contains pair member for continuous BA such as STOP and UNSTOP, which means target enter BA status and leave BA status.
• there is no flag of BA inside vp_frame_target type, all nodes in pipeline need maintain it by themself if they want to know BA status for some task(like counter for crossline).