Parallel optimization fix

[alexfeld]

I discovered that the __getstate__ method in Optimization was reverted to it's original, buggy form. I fixed it and added a new test to make sure that this will show up in the future.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 0.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   85.64%   85.65%   +0.01%     
==========================================
  Files          27       27              
  Lines        2417     2419       +2     
  Branches      388      388              
==========================================
+ Hits         2070     2072       +2     
  Misses        333      333              
  Partials       14       14

Impacted Files	Coverage Δ
paramz/optimization/optimization.py	99.45% <0%> (ø)	⬆️
paramz/model.py	98% <100%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8d83095...e96cd3b. Read the comment docs.