🐛 fix check for density matrices in compute tables

[burgholzer]

Description

This PR fixes an easy to overlook bug in the compute table implementation. The check in the old implementation was never evaluating to true due to the types being used in the various compute tables.
This allowed matrix and density matrix nodes to me mixed because the subsequent condition was never checked.
This PR adjusts the condition so that the checks are evaluated for both, addition and multiplication of density matrices.

Note that this has quite some interesting consequences, where a density matrix can have two successors that have exactly the same structure, but are represented by different DDs due to one (0-successor) representing a density matrix and one (1-successor) representing a matrix. Although both successors have to represent diagonal matrices for this to occur.

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 (6be2196) to head (806283e).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #568   +/-   ##
=====================================
  Coverage   91.1%   91.2%           
=====================================
  Files        132     132           
  Lines      13896   13901    +5     
  Branches    2194    2195    +1     
=====================================
+ Hits       12672   12679    +7     
+ Misses      1224    1222    -2     

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

Files	Coverage Δ
include/mqt-core/dd/ComputeTable.hpp	100.0% <ø> (ø)

... and 2 files with indirect coverage changes