Transform to split sums into single terms

[lillian542]

Context: Some simulators can handle non-commuting observables but don't take multi-term observables.

Description of the Change: We add a transform that splits the observables on a tape up into single-term observables.

[github-actions[bot]]

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[lillian542]

[sc-60187]

[lillian542]

[sc-60188]

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 26 lines in your changes missing coverage. Please review.

Project coverage is 99.60%. Comparing base (ab8b6d5) to head (2c59b61). Report is 19 commits behind head on master.

Files	Patch %	Lines
pennylane/transforms/split_to_single_terms.py	29.41%	24 Missing :warning:
pennylane/transforms/split_non_commuting.py	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5884 +/- ## ========================================== - Coverage 99.68% 99.60% -0.08% ========================================== Files 421 426 +5 Lines 40499 40496 -3 ========================================== - Hits 40370 40336 -34 - Misses 129 160 +31 ```

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