♻️ Eliminate DD terminal nodes

[burgholzer]

Description

This PR completely eliminates the static DD node terminals and replaces them by using nullptr instead.
For one, this eliminates some global static variables.. Yay 😃
Furthermore, DD terminal nodes are no longer associated with the index -1. This allows changing the qubit index to an unsigned type and unlocks double the number of qubits (while also allowing to get rid of some static_casts).
Finally, analysis has shown that the DD node size does not increase when using 16 bits for the qubit index (due to alignment). Hence, the package now directly supports up to 65536 qubits 🥳.

Note that this, naturally, is a breaking change. Qubit indices are now unsigned and checks for a terminal node should now use the respective functions of the Node classes instead of relying the variable index to be -1.
Something to look out for: Any decreasing loops that used a Qubit index for iteration and relied on >=0 stopping conditions have to be revised since they now just wrap around at zero.

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

Merging #381 (6eb2b56) into main (b1c49b9) will increase coverage by 0.0%.
The diff coverage is 97.4%.

@@          Coverage Diff          @@
##            main    #381   +/-   ##
=====================================
  Coverage   89.0%   89.1%           
=====================================
  Files         97      97           
  Lines      11708   11713    +5     
  Branches    2098    2103    +5     
=====================================
+ Hits       10431   10437    +6     
+ Misses      1277    1276    -1     

Impacted Files	Coverage Δ
include/dd/DDDefinitions.hpp	100.0% <ø> (ø)
include/dd/NoiseFunctionality.hpp	93.0% <92.1%> (ø)
src/dd/Simulation.cpp	97.3% <94.1%> (-0.1%)	⬇️
include/dd/Package.hpp	94.5% <97.8%> (+0.3%)	⬆️
include/dd/Export.hpp	94.8% <100.0%> (-1.0%)	⬇️
include/dd/Node.hpp	100.0% <100.0%> (ø)
include/dd/Operations.hpp	83.7% <100.0%> (-0.2%)	⬇️
include/dd/StochasticNoiseOperationTable.hpp	93.5% <100.0%> (ø)
include/dd/UniqueTable.hpp	98.0% <100.0%> (+0.9%)	⬆️
src/dd/Edge.cpp	88.0% <100.0%> (+0.2%)	⬆️
... and 3 more

... and 1 file with indirect coverage changes

[github-actions]

Cpp-Linter Report ✔️

No problems need attention.

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[hillmich]

Thanks a lot for this effort! The increase in available qubits is really nice.

You correctly point out that decrementing loops that check for >=0 require attention. Please wait a bit before merging so I can make this work correctly in the simulator :)

[hillmich]

It took some effort, but the simulator works now :)

[burgholzer]

It took some effort, but the simulator works now :)

Nice. Thanks for your work on that!
I'll probably merge today or tomorrow 😌