* when printing a message, tell the name of the derived class.
* print total power only when state SML_FINAL reached.
* tell if a checksum verification error occurred.
avoid additional conversions and avoid double for the fact that
calculations on type double are implemented in software, whereas
float is handled in hardware on ESP32.
this new class handles SML data. it uses the SML lib to decode values
and manages those. this de-duplicates code as the class is applicable
to all power meters that collect SML data.
