Fix memory leak

[svchb]

We found that not freeing xpvd caused a memory leak in our simulations in which we append to the collection in every write step.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.93%. Comparing base (a7c5e2f) to head (3554127).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #160   +/-   ##
=======================================
  Coverage   96.92%   96.93%           
=======================================
  Files          15       15           
  Lines         879      880    +1     
=======================================
+ Hits          852      853    +1     
  Misses         27       27           

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[jipolanco]

Thank you!