Support for expected values of Circuits

[Mandrenkov]

Context:
One of the barriers to developing a useful PennyLane plugin based on Jet is supporting efficient expected value computations. This PR does just that by implementing a relevant operation in the Circuit class.

Description of the Change:

• The Circuit class now has an expected value operation which accepts an arbitrary observable as input.
• An Adjoint decorator class has been added to replace the adjoint parameter of Gate.tensor().
  • A similar approach may be used in the future to support operator prefactors.
• The Circuit._append_validator() method decorator is now a regular method called Circuit._validate_wire_ids().
  • The resulting code is simpler and iterator-friendly.
• Several docstrings have been fixed to comply with Xanadu OSS conventions.

Benefits:

• It is easy to compute the expected value of an observable with respect to a Circuit instance.
• There exists an efficient implementation of a core PennyLane device operation (i.e., expval()).

Possible Drawbacks:
None.

Related GitHub Issues:
None.

[github-actions]

Test Report (C++) on Ubuntu

    1 files  ±0      1 suites  ±0   0s ⏱️ ±0s
521 tests ±0  521 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
870 runs  ±0  870 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit aa4b4bf. ± Comparison against base commit aa4b4bf.

♻️ This comment has been updated with latest results.

[github-actions]

Test Report (Python) on Ubuntu

    1 files  ±0      1 suites  ±0   6s ⏱️ ±0s
512 tests ±0  512 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit aa4b4bf. ± Comparison against base commit aa4b4bf.

♻️ This comment has been updated with latest results.

[mlxd]

Looks good to me --- just a small query on the use of the Adjoint class, but feel free to ignore.

[Mandrenkov]

Thanks for the review, @mlxd!