Add resonator depletion time to Qubit class

[Jacfomg]

As title says. Related to qiboteam/qibolab_platforms_qrc#135

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.61%. Comparing base (41fcafd) to head (2d8b58d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #895   +/-   ##
=======================================
  Coverage   66.61%   66.61%           
=======================================
  Files          55       55           
  Lines        5942     5943    +1     
=======================================
+ Hits         3958     3959    +1     
  Misses       1984     1984           

Flag	Coverage Δ
unittests	66.61% <100.00%> (+<0.01%)	⬆️

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