[Ovmsdev] Nissan Leaf battery cell temperature and voltage

[Robin O'Leary]

Looking on github, I just noticed that I hadn't posted the separate
changes for the LEAF BMS (disentangled from the wider issue of generalizing
IncomingPollReply).  Sorry about that.  Rather than clutter up the previous
messy branch, I made a new pull request:

https://github.com/openvehicles/Open-Vehicle-Monitoring-System-3/pull/199