🐛 Fix definition issue for DCX gate in DD package

[burgholzer]

Description

Fixes #489.
The DCX gate (as well as the xx_minus_yy gate) had wrong gate matrix definitions in the underlying DD package within mqt-core.
These have now been fixed. A test has been added to check that the proper definition can be successfully verified.

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 95.3%. Comparing base (7f13175) to head (1a30d2f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #494   +/-   ##
=====================================
  Coverage   95.3%   95.3%           
=====================================
  Files         40      40           
  Lines       1835    1835           
  Branches     224     224           
=====================================
+ Hits        1749    1750    +1     
+ Misses        86      85    -1     

Flag	Coverage Δ
cpp	95.8% <ø> (+<0.1%)	⬆️
python	93.7% <ø> (ø)

see 2 files with indirect coverage changes