Added device Pfeiffer TPG controller

PfeifferTPG is a device which uses serial communication. There are many different TPG models, which use mostly the same commands. One exception is the lookup table for the full scale range, which is different between TPG25xA and TPGx6x models. Since this is a minor difference, it seems not worth the trouble of implementing different devices (even using inheritance). Several lookup tables are needed to make the commands more user-friendly. Maybe there are better ways for implementing these lookup tables, and maybe they are not quite where they belong in the code (config or device). Suggestions for improvement?