Store energy in mWs

[mcspr]

subj

Current conversion does truncation

espurna/code/espurna/sensor.h

Lines 86 to 88 in 5f6df82

	constexpr WattSeconds(Watts watts, espurna::duration::Seconds seconds) :
	value(static_cast<Type>(watts.value * seconds.count()))
	{}

https://en.cppreference.com/w/cpp/language/operator_arithmetic#Conversions
https://en.cppreference.com/w/cpp/language/implicit_conversion#Floating.E2.80.93integral_conversions
For example, sensor read 0.9W over 1 second will result in 0. Longer interval will eventually allow for the value to appear, but this may also result in loss of precision when it is truncated to the integer part.

Current Energy pair is kWh + Ws, which could become kWh + mWs instead

espurna/code/espurna/sensor.h

Lines 141 to 144 in 5f6df82

	struct Energy {
	struct Pair {
	KilowattHours kwh;
	WattSeconds ws;

This needs new type and a proper math adjustment.
Plus, everything related to serialization needs to know of these different conversion rules. Right now it only understands kWh + Ws