Added function to return minimum readout hold-off including unit test.

[nielshaandbaek]

Fixes #191.

Changes proposed in this pull request:

• Added minimum_holdoff attribute to UHFQuantumController class.
• Added unit test of minimum_holdoff attribute.
• Attribute returns the minimum hold-off in units of seconds between two readout operations.

@chellings

In order for the pull request to be merged, the following conditions must be met:

• test suite (Github actions) passes
• all reasonable issues raised by codacy must be resolved
• a positive review is required

Whenever possible the pull request should

• follow the PEP8 style guide
• have tests for the code
• be well documented and contain comments

Tests are not mandatory as this is generally hard to make for instruments that interact with hardware.

[codecov-commenter]

Codecov Report

Merging #651 (10908d8) into develop (2d0c14e) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #651      +/-   ##
===========================================
+ Coverage    32.72%   32.75%   +0.02%     
===========================================
  Files          316      316              
  Lines        69212    69235      +23     
===========================================
+ Hits         22652    22676      +24     
+ Misses       46560    46559       -1     

Impacted Files	Coverage Δ
...truments/ZurichInstruments/UHFQuantumController.py	37.69% <100.00%> (+0.63%)	:arrow_up:
...trument_drivers/physical_instruments/test_UHFQC.py	86.84% <100.00%> (+2.30%)	:arrow_up:

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