Spectrum M4i: improve blockavg functionality and add acquisition functions

[peendebak]

This PR does the following:

• Allows the blockavg_hardware_trigger_acquisition to use naverages=1
• Add functions to start data acquisition and retrieve data independently (all functions so far combined these two). Any suggestions for the names start_acquisition and retrieve_data are welcome.
• Put common code into _transfer_buffer_numpy

@jenshnielsen @WilliamHPNielsen

[codecov]

Codecov Report

Merging #970 into master will not change coverage.
The diff coverage is n/a.

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  Coverage   78.61%   78.61%           
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  Files          45       45           
  Lines        6419     6419           
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  Hits         5046     5046           
  Misses       1373     1373

[jenshnielsen]

Looks fine to me but I have no way of testing

I took the liberty to change the name of this pr to include the instrument. Please do this in the future as it makes it much simpler to see what it's about