QCUT: Support returning multiple Pauli words

[trbromley]

Context:

Supports circuits with measurements composed of expectation values of multiple (commuting) Pauli words

Benefits:

Huge savings on the number of executions

Possible Drawbacks:

The change feels a bit hacked-in. TF JIT breaks

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[codecov[bot]]

Codecov Report

Merging #2551 (aef61d9) into master (7575ecb) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2551   +/-   ##
=======================================
  Coverage   99.64%   99.65%           
=======================================
  Files         261      261           
  Lines       21970    22016   +46     
=======================================
+ Hits        21893    21939   +46     
  Misses         77       77           

Impacted Files	Coverage Δ
pennylane/transforms/qcut.py	100.00% <100.00%> (ø)

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[trbromley]

Closing this PR but please do not delete the branch, it is needed for this repo: https://github.com/XanaduAI/randomized-measurements-circuit-cutting