♻️ streamline makeTwoQubitGateDD method

[burgholzer]

Description

The second follow-up PRs to #358, which tries to simplify the underlying functions and methods for representing operations.
This PR specifically streamlines the makeTwoQubitGateDD method in the DD package and eliminates the nqubits and start parameters, which are no longer necessary.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 91.2%. Comparing base (e419f69) to head (2562d24).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #572   +/-   ##
=====================================
  Coverage   91.2%   91.2%           
=====================================
  Files        132     132           
  Lines      13989   13978   -11     
  Branches    2225    2216    -9     
=====================================
- Hits       12766   12760    -6     
+ Misses      1223    1218    -5     

Flag	Coverage Δ
cpp	90.9% <100.0%> (+<0.1%)	⬆️
python	99.5% <ø> (ø)

Files	Coverage Δ
include/mqt-core/dd/Operations.hpp	88.2% <100.0%> (-0.2%)	⬇️
include/mqt-core/dd/Package.hpp	97.6% <100.0%> (+0.4%)	⬆️

... and 1 file with indirect coverage changes