Implement amplitude damping noise model

[ahkatlio]

Description of the change:
Added amplitude damping noise support for pattern simulation with the densitymatrix backend. This includes two new channel functions in channels.py, two noise classes, and a new AmplitudeDampingNoiseModel that works the same way as DepolarisingNoiseModel so it should be familiar to anyone who already used that. Also added 27 tests and updated the __init__ exports.

I also added a line to CHANGELOG.md under [unreleased].

Related issue:
Closes #497

[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.91%. Comparing base (fcdb8f4) to head (c9246c2).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
graphix/noise_models/amplitude_damping.py	92.42%	5 Missing ⚠️
graphix/channels.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #527      +/-   ##
==========================================
+ Coverage   88.85%   88.91%   +0.05%     
==========================================
  Files          49       50       +1     
  Lines        7135     7215      +80     
==========================================
+ Hits         6340     6415      +75     
- Misses        795      800       +5     

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[pranav97nair]

Hello @ahkatlio , thanks for your submission! However, as per the acceptance criteria stated in Issue #497, we cannot review your PR until the CI is passing in Github. Please merge the master branch into your fork and address the currently failing checks.