Add Umbrella Sampling example notebook

[hejung]

Example application in which we progressively populate the windows of an Umbrella Sampling simulation along a 1D collective variable. In this example we use capped alanine dipeptide and bias along the ψ dihedral.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.08%. Comparing base (ea60cd7) to head (523d2a4).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

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##             main      #51   +/-   ##
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  Coverage   79.08%   79.08%           
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  Files          23       23           
  Lines        2462     2462           
  Branches      318      318           
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  Hits         1947     1947           
  Misses        421      421           
  Partials       94       94           

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