Add `save_optimization` function in workflows

[goerz]

Although usually, @optimize_or_load is better, sometimes it's easiest to use optimize in the REPL, but then save the result manually.

[codecov[bot]]

Codecov Report

Merging #35 (5f40aef) into master (b2cd0eb) will increase coverage by 0.58%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage   74.41%   75.00%   +0.58%     
==========================================
  Files           5        5              
  Lines         297      304       +7     
==========================================
+ Hits          221      228       +7     
  Misses         76       76              

Impacted Files	Coverage Δ
src/workflows.jl	86.20% <100.00%> (+1.20%)	:arrow_up:

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