Clean up preconditioner code

[ChrisRackauckas]

No description provided.

[codecov]

Codecov Report

Merging #66 (2ca92ac) into main (8194943) will decrease coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
- Coverage   79.39%   79.00%   -0.40%     
==========================================
  Files           7        7              
  Lines         398      381      -17     
==========================================
- Hits          316      301      -15     
+ Misses         82       80       -2     

Impacted Files	Coverage Δ
src/common.jl	62.79% <ø> (-1.66%)	⬇️
src/preconditioners.jl	86.11% <ø> (+1.66%)	⬆️
src/iterative_wrappers.jl	91.50% <100.00%> (-0.46%)	⬇️

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