✨ added state preparation routines for GHZ and W state

[BertiFlorea]

Description

In #341 the DD package provided methods for directly constructing DD for computational basis states or any product state that can be constructed from the single-qubit states |0>, |1>, |+>, |->, |L>, |R>.

Updates

There are now 2 routines which implement the

• GHZ state
• W state,
  also, there are unit tests which check that the preparation routines provide the desired states for a smaller number of qubits.

Fixes #341

Notes

In the beginning, I tried adding the routines to a different header file, however, I noticed that it was a lot easier to stick to the current structure and add the functions to the Package.hpp header inside the DD package. Additionally, I tried to use the existing functions in order to create & normalize the decision diagrams.

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 #445 (ae4679c) into main (6809ce2) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #445   +/-   ##
=====================================
  Coverage   90.1%   90.1%           
=====================================
  Files        111     111           
  Lines      11817   11845   +28     
  Branches    2057    2065    +8     
=====================================
+ Hits       10649   10677   +28     
  Misses      1168    1168           

Flag	Coverage Δ
cpp	89.9% <100.0%> (+<0.1%)	⬆️
python	100.0% <ø> (ø)

Files	Coverage Δ
include/dd/Package.hpp	94.9% <100.0%> (+0.1%)	⬆️

[burgholzer]

Hey 👋🏼

Many thanks for this nice initial contribution. I just went through your changes and left some comments on things that I believe still need a more detailed look/some changes.

Note that #444 will hopefully land soon and that might require some changes within this PR. No action item for now though.

[burgholzer]

Many thanks for the changes here! Looking way better already 👍🏼
I left some comments inline on how to improve it even further and a minor request for some more generic tests that I think would make sense.

[burgholzer]

Many thanks for the latest set of changes. We are really getting there. I think this should be the last round of changes requested. After that, we are good to merge.

[burgholzer]

Many thanks for this nice addition!