Allow modifying Julia's optim pipeline

[jpsamaroo]

Closes #23

[codecov]

Codecov Report

Merging #30 into master will increase coverage by 0.38%.
The diff coverage is 80.32%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   72.14%   72.53%   +0.38%     
==========================================
  Files          18       18              
  Lines        1156     1216      +60     
==========================================
+ Hits          834      882      +48     
- Misses        322      334      +12     

Impacted Files	Coverage Δ
src/gcn.jl	54.54% <79.66%> (+23.90%)	⬆️
src/optim.jl	98.55% <100.00%> (+0.02%)	⬆️

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[jpsamaroo]

Not really necessary any longer since #47 fixed the main issue for GCN.