add support for defining threshold for 'no trigger rate'

Created by: andreh12

When data taking and trigger is configured to record splashes (like now), there are typically extended periods with no triggers at all (even the calibration sequence is turned off) to make sure the splashes are (almost) the only triggers which fire, the shift crew can immediately check the event display and splashes are not lost due to inhibiting triggers during the calibration sequence.

However, this leads to a somewhat annoying rate of sound alerts about 'no rate when expected' in the control room :-)

As discussed with @dinyar on the phone, a simple solution for this case would be to introduce an easily configurable parameter defining what should be considered 'zero' in the class NoRate. This parameter could temporarily be set to minus one during splash data taking.

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As a bonus, the DAQExpert could have a special 'splashes' mode which would generate a sound every time the number of triggers increases by a (configurable) number between snapshots (see also https://github.com/cmsdaq/DAQExpert/issues/29#issuecomment-271580418 and #50) but that should be a discussed in a separate issue.