Add trace manipulation commands to Keysight PNA driver

[spauka]

This PR adds trace manipulation commands to the Keysight PNA driver.

Specifically, we now allow the following interactions:

• trace = pna.add_trace(): Create a new trace (of any number) on the PNA
• trace = pna.enable_trace(trace_num): Enable and return the given trace number
• trace.disable(): Disable a trace on the PNA

[codecov]

Codecov Report

Attention: Patch coverage is 17.64706% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 67.50%. Comparing base (277675d) to head (6efabb5).
Report is 52 commits behind head on main.

Files	Patch %	Lines
src/qcodes/instrument_drivers/Keysight/N52xx.py	17.64%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6035      +/-   ##
==========================================
- Coverage   67.52%   67.50%   -0.03%     
==========================================
  Files         350      350              
  Lines       30301    30318      +17     
==========================================
+ Hits        20462    20465       +3     
- Misses       9839     9853      +14     

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