[chbot] PID controller

[Robin Gilks]

Many thanks for the feedback last night - all working now ;)

   - Fixed minor code bug
   - turned on 'reverse' mode on PID to invert output w.r.t. input
   - changed update period from 100ms to 10ms
   - enabled non-zero integrator term to stabilise the feedback loop

Derivative term still causes instability so might need some more tuning but
now a step change settles in < 100ms with no overshoot, the output
converges to less than 1% of the set target voltage without any 'magic
number' hacks

-- 
Robin Gilks
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