This is an effort trying to improve read_mixin.py. This is related to…

… PR #252 where Trendlog creation was clunky and subject to bugs. Old implementation was duplicating code between RPM and RP devices which was complicating code. I'm trying to remove duplicate code by using mixin classes. It is not perfect but at least, we won't have to make double code correction to similar functions. I don't like how _process_new_objects function needs to be handled but for now, BAC0 works and devices get created. I'm really suffering from a bad comprhesnion of segmentation and read property multiple support of old devices... but for now I have to live with what I did in the past.