Relax potential by evolving phi rather than vort

[bendudson]

Adds a switch evolve_vorticity that is true by default, and equations are evolved for both phi and vort.
Setting evolve_vorticity to false modifies the equations so only phi is evolved and vort is calculated from phi. This reduces the size of the system and reduces the order of the PDE. Exploring whether this helps solve transport with drifts faster.

[codecov]

Codecov Report

❌ Patch coverage is 58.74587% with 125 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.52%. Comparing base (7b61dd8) to head (0730073).

Files with missing lines	Patch %	Lines
src/relax_potential.cxx	58.74%	87 Missing and 38 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #570      +/-   ##
==========================================
+ Coverage   59.75%   60.52%   +0.77%     
==========================================
  Files          98       98              
  Lines       10274    10323      +49     
  Branches     1482     1488       +6     
==========================================
+ Hits         6139     6248     +109     
+ Misses       3488     3400      -88     
- Partials      647      675      +28     

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