🚸🚨 Eliminate reference members and improve TaskManager interface

[burgholzer]

Description

This was triggered by a clang-tidy warning that popped up in #348.
This PR also adjusts the TaskManager interface to take a reference to a package instance instead of a unique pointer; making the interface more generic.
This should unblock #348.

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 ✅

Comparison is base (1ab1ffa) 96.2% compared to head (b112d72) 96.3%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #349   +/-   ##
=====================================
  Coverage   96.2%   96.3%           
=====================================
  Files         34      34           
  Lines       1749    1751    +2     
  Branches     214     214           
=====================================
+ Hits        1684    1687    +3     
+ Misses        65      64    -1     

Flag	Coverage Δ
cpp	96.0% <100.0%> (+<0.1%)	⬆️
python	97.2% <ø> (ø)

Files	Coverage Δ
include/checker/dd/DDEquivalenceChecker.hpp	100.0% <100.0%> (ø)
include/checker/dd/TaskManager.hpp	98.4% <100.0%> (+<0.1%)	⬆️
...checker/dd/applicationscheme/ApplicationScheme.hpp	100.0% <100.0%> (ø)
...dd/applicationscheme/GateCostApplicationScheme.hpp	95.2% <100.0%> (ø)
...d/applicationscheme/LookaheadApplicationScheme.hpp	100.0% <ø> (ø)
...pplicationscheme/ProportionalApplicationScheme.hpp	100.0% <ø> (ø)

... and 2 files with indirect coverage changes