Adding propagator struct

[Gavin-Rockwood]

Checklist

Thank you for contributing to QuantumToolbox.jl! Please make sure you have finished the following tasks before opening the PR.

• Please read Contributing to Quantum Toolbox in Julia.
• Any code changes were done in a way that does not break public API.
• Appropriate tests were added and tested locally by running: make test.
• Any code changes should be julia formatted by running: make format.
• All documents (in docs/ folder) related to code changes were updated and able to build locally by running: make docs.
• (If necessary) the CHANGELOG.md should be updated (regarding to the code changes) and built by running: make changelog.

Request for a review after you have completed all the tasks. If you have not finished them all, you can also open a Draft Pull Request to let the others know this on-going work.

Description

Describe the proposed change here.

Related issues or PRs

Please mention the related issues or PRs here. If the PR fixes an issue, use the keyword close/closes/closed/fix/fixes/fixed/resolve/resolves/resolved followed by the issue id, e.g. fix #[id]

Additional context

[codecov]

Codecov Report

❌ Patch coverage is 0% with 84 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.15%. Comparing base (9f9da52) to head (c89e3dd).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/time_evolution/propagator.jl	0.00%	84 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (9f9da52) and HEAD (c89e3dd). Click for more details.

HEAD has 7 uploads less than BASE

Flag	BASE (9f9da52)	HEAD (c89e3dd)
	12	5

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #660       +/-   ##
===========================================
- Coverage   93.02%   32.15%   -60.87%     
===========================================
  Files          57       58        +1     
  Lines        3726     4742     +1016     
===========================================
- Hits         3466     1525     -1941     
- Misses        260     3217     +2957     

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