Update decomposition function

[jama7168]

Before submitting

Please complete the following checklist when submitting a PR:

• All new features must include a unit test.
  If you've fixed a bug or added code that should be tested, add a test to the
  test directory!

• All new functions and code must be clearly commented and documented.
  If you do make documentation changes, make sure that the docs build and
  render correctly by running make docs.

• Ensure that the test suite passes, by running make test.

• Ensure that code and tests are properly formatted, by running make format or black -l 100 <filename> on any relevant files. You will need to have the Black code format installed:
  pip install black.

• Add a new entry to the .github/CHANGELOG.md file, summarizing the
  change, and including a link back to the PR.

• The Strawberry Fields source code conforms to
  PEP8 standards.
  We check all of our code against Pylint.
  To lint modified files, simply pip install pylint, and then
  run pylint strawberryfields/path/to/file.py.

When all the above are checked, delete everything above the dashed
line and fill in the pull request template.

---

Context:
During testing the interferometer Class I came across the problem that Interferometer(unitary_matrix, mesh = 'triangular', tol = 1e-10) and Interferometer(unitary_matrix, mesh = 'rectangular_phase_end', tol = 1e-10) give different results when applied within the boson sampling. As the matrix we are starting with is the same that should not be (see also here: https://discuss.pennylane.ai/t/problems-in-understanding-sf-decompositions-triangular/7875/6 for more details and the discussion).

Description of the Change:
I changed the following things:
the order of the return in the triangular decomposition from reversed(tlist), np,diag(localV), None to None, np,diag(localV), t_list so that in the Interferometer_decomposition function the order of application is first the diagonal (phase shifters) and then the T_i[0] to T_i[N].
Then I changed the interferometer_decompose method such that it checks if the BS1 from the Decomposition is None.
I also provided the minimal example as a test with which i tested the interferometer class. Now the results for both are the same as it should be.

Benefits:

Fixed a problem with the triangular decomposition in the Interferometer.

Possible Drawbacks:
Since I changed the return of the triangular decomposition it might affect other functions. Maybe we should check that first!!

Related GitHub Issues:
Not a gitub Issue but here the form discussion I had:
https://discuss.pennylane.ai/t/problems-in-understanding-sf-decompositions-triangular/7875/6

[CatalinaAlbornoz]

Hi @jama7168,

Thanks again for opening this PR. A member of our team is currently reviewing it. We should get back to you soon with next steps.

Have a good weekend!