Do not dealloc the scratch space

[drhagen]

This is a memory bug in the compute/assemble kernels. (The evaluate kernel is unaffected.) A recent PR (#99) called realloc at the end of the assemble/evaluate kernels to deallocate any unused space. This is technically wrong in the assemble/compute case because compute still writes 0s to the scratch space, even if that space is ultimately not used (because there were no nonzeros to write to it). These zeros may be written in compute after being deallocated in assemble.

This PR fixes this by always leaving the scratch space allocated. This is slightly memory wasteful for evaluate kernels, but the alternative is to either desync how evaluate and assemble work or move the scratch space to some temporary location and only copy the result to vals if it was written to.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.79%. Comparing base (3a9fbb5) to head (e671dfc).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   90.78%   90.79%   +0.01%     
==========================================
  Files          60       60              
  Lines        3179     3183       +4     
  Branches      389      389              
==========================================
+ Hits         2886     2890       +4     
  Misses        209      209              
  Partials       84       84              

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