Writes time out on APSystems ECU-R because it only supports multi-register writes (0x10), not single-register writes (0x06)

[MartijnVdS]

I've been trying to set WMaxLimPct (part of model 123, immediate controls) on my APSystems ECU-R / YC600 and it does not work.

model.WMaxLimPct.cvalue = 100.0
model.write() # this gives a timeout

The problem seems to be that the other end does not support function code 0x06 (single-register write), but only 0x10 (multi-register write), while pysunspec2 contains some logic to use single-register writes if only a single change was done on the model.

By manually disabling the check (changing if count == 1: in sunspec2/modbus/modbus.py line 781 to if False), writes always use function 0x10 and they succeed; values get updated correctly.