Allow init argument for sum

[blegat]

Closes #305

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.34%. Comparing base (93c7e3b) to head (978e294).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
- Coverage   91.85%   90.34%   -1.51%     
==========================================
  Files          23       22       -1     
  Lines        2246     2227      -19     
==========================================
- Hits         2063     2012      -51     
- Misses        183      215      +32     

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[odow]

I'll add a test