implement a function which allows to reset the SML decoder. this new
function is used after a datagram ends. for the SML HTTP power meter
this is simple: after all bytes from the request's answer have been
decoded, we reset the decoder. for the SML serial power meter, we
perform the reset after a datagram ended based on timing (no new bytes
have been received for a specific amount of time).
cache the values decoded in the SML datagram and only copy them to the
local stash of values if the checksum of the SML datagram matched. also
makes sure that values from incomplete SML datagrams are not used.
moreover, we now only publish values to the MQTT broker that we actually
decoded (successfully) from an SML datagram (we previously published 0.0
as values to topics we never decoded a value for).
* 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.
