Move JLD2 dependency to package extension

[simsurace]

Analogous to JuliaGraphs/MetaGraphs.jl#50 and JuliaGraphs/StaticGraphs.jl#43

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.96%. Comparing base (535ecbb) to head (20979b2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   89.77%   89.96%   +0.19%     
==========================================
  Files           6        8       +2     
  Lines         313      329      +16     
==========================================
+ Hits          281      296      +15     
- Misses         32       33       +1     

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