driver/sigrokdriver: add channel group parameter

[nlabriet]

Add channel group parameter to sigrok devices

Description

Trying to interface Labgrid with a power supply using sigrok, I need to specify the channel group to be used (in my case: -g 1).

Checklist

• The arguments and description in doc/configuration.rst have been updated
• PR has been tested with a scpi-pps power supply

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 56.5%. Comparing base (0304ec6) to head (d6c5973).

Files with missing lines	Patch %	Lines
labgrid/driver/sigrokdriver.py	0.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1536     +/-   ##
========================================
- Coverage    56.5%   56.5%   -0.1%     
========================================
  Files         168     168             
  Lines       13111   13118      +7     
========================================
+ Hits         7417    7420      +3     
- Misses       5694    5698      +4     

Flag	Coverage Δ
3.10	56.5% <42.8%> (-0.1%)	⬇️
3.11	56.5% <42.8%> (-0.1%)	⬇️
3.12	56.5% <42.8%> (-0.1%)	⬇️
3.13	56.5% <42.8%> (-0.1%)	⬇️
3.9	56.5% <42.8%> (-0.1%)	⬇️

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