dmm/eev121gw: introduce parser for EEVblog 121GW 19-byte binary packets
Introduce the dmm/eev121gw.c source file with parse routines for the
EEVblog 121GW meter's 19-bytes binary packets. Get the values and MQ
properties of the device's several displays (main, sub, bar) in several
individual parse calls.
This commit introduces initial support for the device. Some of the modes
and features are untested, as are some of the device's ranges.