[unitaryHACK] Deprecated qml.inv()

[tgag17]

Context:
Deprecated qml.inv().

Description of the Change:
Added warnings in the documentation and code

Benefits:
Want to shift users to qml.adjoint() as it is a more general function

Related GitHub Issues:
Fixes #1195

[codecov]

Codecov Report

Merging #1325 (4c91803) into master (9777f09) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1325   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files         148      148           
  Lines       11450    11452    +2     
=======================================
+ Hits        11240    11242    +2     
  Misses        210      210           

Impacted Files	Coverage Δ
pennylane/utils.py	96.00% <100.00%> (+0.05%)	⬆️

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[josh146]

Thanks for this contribution @tgag17! 🎉

Just two small things before this can be merged in:

• Don't forget to add a link/description of this change to the changelog! And to add your name to the 'Contributors' section 🙂

• It would be good to add a test to ensure that the warning is working correctly. This test can be added to tests/test_utils.py, to the TestInv class. In particular, the

  with pytest.warns(UserWarning, match="warning message"):
      #code that raises a warning

  context manager allows easy testing of whether warnings are raised. The PyTest documentation has some more info here: https://docs.pytest.org/en/stable/warnings.html#warns

[tgag17]

Thanks for the review and helping out with the tests @josh146! I have made the required commits

[josh146]

Thanks for this great contribution @tgag17! This is merge ready from my end 🙂