CMP #5: Add Sample MP class

[AlbertMitjans]

1. Create _Sample class (name will be changed to Sample after the whole refactor).
2. Return _Sample instance in qml.sample.
3. Copy code from QubitDevice.sample into _Sample.process + improve code style.
4. Add teardown_method to the test_sample.py tests that checks that QubitDevice.sample and _Sample.process return the same values for all tests.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #3288 (1ccfe41) into master (dae6540) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3288   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files         298      298           
  Lines       26163    26188   +25     
=======================================
+ Hits        26085    26110   +25     
  Misses         78       78           

Impacted Files	Coverage Δ
pennylane/measurements/measurements.py	100.00% <100.00%> (ø)
pennylane/measurements/mutual_info.py	100.00% <100.00%> (ø)
pennylane/measurements/sample.py	100.00% <100.00%> (ø)
pennylane/measurements/state.py	100.00% <100.00%> (ø)
pennylane/measurements/vn_entropy.py	100.00% <100.00%> (ø)

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[eddddddy]

Looks good @AlbertMitjans 💯