S0 should not consume two impulses per reading

[andig]

when adding calculated instant power readings to the output of the s0 meter,
it was made to consume two pulses in MeterS0::read to calculate power.
this is not necessary, we can simply remember the timestamp of the previous reading's pulse.

Solved as part of #110, this is just a reminder.

[r00t-]

https://github.com/volkszaehler/vzlogger/blob/master/src/protocols/MeterS0.cpp#L115