* add support for energy & power readings on SML based power meters, taking OBIS 16.7.1 for power (using mod. SML Parser lib. by olliiiver)
* switched SML read to use software serial
* made total power meter response controled by meter source to obtain either the sum of phase powers or explicit total power provided by meter
* made mqtt subscriptions to power meter topics meter source dependend
* simplified SML read loop and OBIS handler registration, + minor refactoring
* minor cleanup/style changes and optim. PowerMeter
* fixed build, add SOURCE_SML == 4
* removed optional usage of HW serial for SML power meter
* switched to usage of _powerMeter1Power for SML power reading to allign better with existing code
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Co-authored-by: helgeerbe <helge@erbehome.de>
