Adding missing <sstream> includes

[jonoomph]

They should be included - and were not failing on more modern compilers, so a little hard to catch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.64%. Comparing base (13e1d9d) to head (16f791a).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1024      +/-   ##
===========================================
+ Coverage    59.60%   59.64%   +0.04%     
===========================================
  Files          206      206              
  Lines        20374    20374              
===========================================
+ Hits         12143    12152       +9     
+ Misses        8231     8222       -9     

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