♻️ Revised qubit register handling

[burgholzer]

Description

This kind of supersedes #493 and provides an alternative, less intrusive version that properly handles ancillary qubits and marking qubits in existing registers as ancillaries.
It simply checks on the removal of a qubit from a qreg whether that qubit is marked as ancillary and correspondingly decrements the right qubit number counter.

This has also been tested in QCEC to resolve the errors that were popping up over there.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

Merging #495 (89f2885) into main (3759199) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #495     +/-   ##
=======================================
- Coverage   90.3%   90.3%   -0.1%     
=======================================
  Files        111     111             
  Lines      11829   11826      -3     
  Branches    2057    2057             
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- Hits       10690   10682      -8     
- Misses      1139    1144      +5     

Flag	Coverage Δ
cpp	90.1% <100.0%> (-0.1%)	⬇️
python	100.0% <ø> (ø)

Files	Coverage Δ
src/QuantumComputation.cpp	81.3% <100.0%> (-1.0%)	⬇️