Add new method repeat_full to VariableList

[macladson]

While working on sigp/lighthouse#8032, there are a couple of situations where we initialize a VariableList into a full list of all 0's. Having to handle errors in this case when none can occur is a bit annoying so a dedicated function to do this helps to avoid an ugly expect. This was suggested by Michael here: sigp/lighthouse#8032 (comment)

This is the VariableList equivalent of the from_elem method on FixedVector. At a later time it might be worth considering renaming the from_elem method to repeat for consistency.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.45%. Comparing base (44deb75) to head (424c60e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   58.84%   59.45%   +0.61%     
==========================================
  Files           8        8              
  Lines         294      296       +2     
==========================================
+ Hits          173      176       +3     
+ Misses        121      120       -1     

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