Prepare integration with DiffOpt

[joaquimg]

replaces #143

This will be associated with a PR in DiffOpt.

The main goal is to keep POI as simple and low-weight as possible.
This way, the responsibility for understanding differentiation falls on DiffOpt and its maintainers.

[codecov]

Codecov Report

Attention: Patch coverage is 95.77465% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.39%. Comparing base (18401ef) to head (bb7ff58).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/MOI_wrapper.jl	94.59%	2 Missing ⚠️
src/parametric_functions.jl	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
+ Coverage   95.15%   95.39%   +0.24%     
==========================================
  Files           5        5              
  Lines        1031     1085      +54     
==========================================
+ Hits          981     1035      +54     
  Misses         50       50              

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