WIP: Allowing the merging of samples for displaying model predictions without touching the PyHF model

[MoAly98]

Addresses #502

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (ffa5ca2) to head (548ec1f).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #505   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           22        22           
  Lines         2094      2145   +51     
  Branches       347       356    +9     
=========================================
+ Hits          2094      2145   +51     

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[MoAly98]

@alexander-held -- I think this is a minimally invasive solution to the code, and I think I have added tests to cover the changes. Let me know if you have a test setup that you would like me to try out. I also appreciate feedback on performance aspects, especially on memory consumption.