Create SensorModel and use throughout library

[bachya]

Describe what the PR does:

Until now, we returned sensors as dictionaries. This PR introduces SensorModel and uses it throughout the library.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #14 (451b906) into dev (085d0b9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##               dev       #14    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           11        11            
  Lines          246       416   +170     
==========================================
+ Hits           246       416   +170     

Impacted Files	Coverage Δ
aiopurpleair/const.py	100.00% <100.00%> (ø)
aiopurpleair/helpers/validators/sensors.py	100.00% <100.00%> (ø)
aiopurpleair/models/sensors.py	100.00% <100.00%> (ø)

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[codeclimate]

Code Climate has analyzed commit 451b906 and detected 0 issues on this pull request.

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