Use fixed PauliString.after() in dynamical decoupling.

[babacry]

Context: #7065 fixed PauliString's conjugated_by, before and after. Now dd could use a more reliable after() in pulling Paulis through Cliffords.

Also read through dynamical_decoupling.py and did a couple of nit fixes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.15%. Comparing base (22ed25e) to head (6821cbb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7103      +/-   ##
==========================================
- Coverage   98.15%   98.15%   -0.01%     
==========================================
  Files        1089     1089              
  Lines       95253    95251       -2     
==========================================
- Hits        93500    93497       -3     
- Misses       1753     1754       +1     

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[babacry]

@eliottrosenberg FYI, I've rewrittenPualiString.after(), where the previous implementation caused the dynamical decoupling failures (we used a workaround at that time to fix, which wasn't reliable).

[NoureldinYosri]

nice work @babacry