Add the option to set pulse modulation in the Keysight N51x1 signal generators

[bguttel]

Notice that there are two parameters there, one that sets the general state of the pulse modulation (boolean) and one that sets whether it will be 'internal' or 'external'.

Not all the signal generators come with this feature so I added an if condition to test for it.

[codecov]

Codecov Report

Merging #4840 (8f3685f) into master (0556305) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4840      +/-   ##
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- Coverage   68.30%   68.29%   -0.01%     
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  Files         339      339              
  Lines       32123    32126       +3     
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  Hits        21942    21942              
- Misses      10181    10184       +3     

[jenshnielsen]

@bguttel Thanks, looks good. Could you create a file at /docs/changes/news_fragments/4840.improved_driver with a short description of the fix so it makes it into the changelog

[bguttel]

@bguttel Thanks, looks good. Could you create a file at /docs/changes/news_fragments/4840.improved_driver with a short description of the fix so it makes it into the changelog

Added the file.
@jenshnielsen let me know if there is anything more that you think I should add! :)