* 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.
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.
it is important to separate the capabilities of each power meter
provider into their own class/source file, as the providers work
fundamentally different and their implementations must not be
intermangled, which made maintenance and improvements a nightmare
in the past.
