the SMA power meter implementation did not announce that it received a
new datum, such that the power meter data age was never reset. this made
the power meter values outdated and hence invalid, even though new
values were received and processed.
"powertotal" is always published and it is published by the base class
directly. other values are still published by the derived classes, but
use a base class method, which takes care that a common base topic is
used in particular.
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.
