Add underdamped bosonic spectral density

[cerisola]

This pr adds a convenience constructor for underdamped spectral densities frequently used in the literature.

The pr adds the new bath constructors, documentation, and tests, so I think should have all necessary features to be merged.

The implementation follows exactly the same convention as in QuTiP. I have further tested the results with those of QuTiP and they agree.

Fixes #123

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.85%. Comparing base (7e6a013) to head (f416315).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
- Coverage   91.60%   89.85%   -1.76%     
==========================================
  Files          23       24       +1     
  Lines        1465     1478      +13     
==========================================
- Hits         1342     1328      -14     
- Misses        123      150      +27     

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[ytdHuang]

Thank you for the contribution !

The content looks good to me.

Just a small problem is that the code is not Julia formatted yet, this can be done by simply running the command make format in your local PC. If you need more information, please refer to Contributing to QuantumToolbox.jl - Julia Code Format

[cerisola]

Code formatting should be good now.