Now read out the actual trigger channel after the setting process, and
use this one as default channel for waveform information (WAV:COUNt?
polling in the event loop) and for the TTAGs information.
Since the TTAG are trigger time difference, I'm not sure what it looks
like on non-trigger channels (the trigger channel is not always the
first in the readout list).

The default channel setting is now done at the start of each run (ie
after the data fetch and writing from any channel occured at the end of
the previous run).

Improves the readout settings that was changed in #5911603e