Fix the hotfix: Single unsplit section

[hay-k]

#848 introduced the section for unsplit channels, but it would face issues if a single experiment applies long flux pulses on multiple places (e.g. on qubit and coupler flux channels). The issue being the non-uniqueness of the section name. All unsplit channels should just constitute a single section. Additionally, the pulse start time is accounted for as well, although there is no known use case for it.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[Jacfomg]

Great

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.66%. Comparing base (48d0352) to head (fa9d144).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #858   +/-   ##
=======================================
  Coverage   66.66%   66.66%           
=======================================
  Files          55       55           
  Lines        5900     5901    +1     
=======================================
+ Hits         3933     3934    +1     
  Misses       1967     1967           

Flag	Coverage Δ
unittests	66.66% <100.00%> (+<0.01%)	⬆️

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[alecandido]

@hay-k would you add a test, failing for main and passing for this branch?

This would prevent a possible regression.