use CLOCK_MONOTONIC instead of CLOCK_REALTIME for timer setup

CLOCK_REALTIME is a wall clock, that could change back and forth (user
changes system time, winter/summer time, NTP corrections and so on).
For real-time application CLOCK_MONOTONIC should be used, it doesn't
have such jumps.

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<?xml version="1.0" encoding="UTF-8"?>

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<BaseParams Name="py_ext_0" IEC_Channel="0"/>