vorticity: precalculate Grad(phi)

[malamast]

Minor change in vorticity component

precalculate Grad(phi) instead of recalculating it for every species.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.41%. Comparing base (e0f1ebf) to head (de487f8).

Files with missing lines	Patch %	Lines
src/vorticity.cxx	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
- Coverage   26.41%   26.41%   -0.01%     
==========================================
  Files          90       90              
  Lines        8105     8106       +1     
  Branches     1133     1134       +1     
==========================================
  Hits         2141     2141              
- Misses       5745     5746       +1     
  Partials      219      219              

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