♻️ Eliminate nqubits and startingQubit from Operation class hierarchy

[burgholzer]

Description

The third follow-up PRs to #358, which tries to simplify the Operation class hierarchy by eliminating the nqubits and startingQubit members, which were purely in place for managing the corresponding decision diagrams.

This also practically fixes #345. Only state creation routines now have the additional starting qubit parameter.

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

Attention: Patch coverage is 90.96386% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 91.2%. Comparing base (16ae2b7) to head (1b251aa).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #574     +/-   ##
=======================================
- Coverage   91.2%   91.2%   -0.1%     
=======================================
  Files        132     132             
  Lines      13978   13904     -74     
  Branches    2216    2203     -13     
=======================================
- Hits       12760   12692     -68     
+ Misses      1218    1212      -6     

Flag	Coverage Δ
cpp	90.9% <90.5%> (-0.1%)	⬇️
python	99.5% <100.0%> (ø)

Files	Coverage Δ
include/mqt-core/QuantumComputation.hpp	91.7% <100.0%> (-0.1%)	⬇️
include/mqt-core/dd/Operations.hpp	88.6% <ø> (+0.3%)	⬆️
include/mqt-core/ecc/Ecc.hpp	92.5% <100.0%> (ø)
...mqt-core/operations/ClassicControlledOperation.hpp	96.6% <ø> (-0.3%)	⬇️
include/mqt-core/operations/CompoundOperation.hpp	100.0% <ø> (ø)
...nclude/mqt-core/operations/NonUnitaryOperation.hpp	91.6% <ø> (ø)
include/mqt-core/operations/Operation.hpp	80.7% <100.0%> (+0.2%)	⬆️
include/mqt-core/operations/StandardOperation.hpp	100.0% <ø> (ø)
include/mqt-core/operations/SymbolicOperation.hpp	100.0% <ø> (ø)
src/QuantumComputation.cpp	81.8% <100.0%> (-0.2%)	⬇️
... and 18 more