Add options to recompute gradient and/or obj or not on first iteration

[MaxenceGollier]

closes #278.

[MaxenceGollier]

@dpo, Ideally, i would like to add unit tests but I would like to refactor them first, they are not maintanable as such in my opinion. Could you please take a look at #271 ?

This is blocking for the notion points for this week, and so is #279.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.66%. Comparing base (e0f214d) to head (7540372).
⚠️ Report is 265 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #280       +/-   ##
===========================================
+ Coverage   61.53%   84.66%   +23.13%     
===========================================
  Files          11       13        +2     
  Lines        1292     1624      +332     
===========================================
+ Hits          795     1375      +580     
+ Misses        497      249      -248     

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[dpo]

This looks good! Did you at least test it with your solver?

[MaxenceGollier]

Benchmarks are running: MaxenceGollier/Penelopt.jl#47 i will need a release for the tests to pass.

Edit: benchmarks give a good improvement accross all measures, I will merge like so and add tests afterwards.