WIP: 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 20.00000% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.66%. Comparing base (2eeda48) to head (71d2c00).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/relax_potential.cxx	20.00%	32 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #570      +/-   ##
==========================================
+ Coverage   48.64%   48.66%   +0.01%     
==========================================
  Files          96       96              
  Lines        9887     9896       +9     
  Branches     1435     1437       +2     
==========================================
+ Hits         4810     4816       +6     
+ Misses       4586     4585       -1     
- Partials      491      495       +4     

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