when performing a test request using the web UI, we need to init() the
respective power meter, but we do not want to start the polling task.
hence we move initialization of the polling task to the poll() function.
it will return if the task is setup already, otherwise setup the task.
a 9600 baud serial interface does not need a hardware UART. these
changes switch the SDM power meter implementation to use a software
serial instance instead. this is desirable as hardware UARTs are scarce
and users need them for JK BMS and VE.Direct interfaces.
the destructor will block for way too long if we keep holding the
polling mutex while performing a transcation with the SDM power meter.
when reading, we now release the lock. afterwards, i.e., in between
transactions, we check the stop flag so the task terminates in a timely
manner once asked to do so.
instead of reading the main config's powermeter struct(s), the
individual power meters now are instanciated using a copy of their
respective config. this allows to instanciate different power meters
with different configs. as a first step, this simplifies instanciating
power meters for test purposes.
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.
