weak separation oracle

[matbesancon]

introduces weak separation oracles as an abstract interface that other FW algorithms will be able to leverage.

Right now the only weak oracles are cache-based (active set or cache) in the various algorithms. This generalizes the concept to accept oracles including:

• heuristics (no guarantee on the solution)
• approximation algorithms (providing an optimality bound)

[matbesancon]

Still to do: introduce and exploit in the lazified algorithms

[codecov]

Codecov Report

Attention: Patch coverage is 87.38739% with 14 lines in your changes missing coverage. Please review.

Project coverage is 86.85%. Comparing base (d7be106) to head (fb6773a).
Report is 120 commits behind head on master.

Files	Patch %	Lines
src/abstract_oracles.jl	77.77%	12 Missing ⚠️
src/pairwise.jl	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
- Coverage   87.03%   86.85%   -0.18%     
==========================================
  Files          20       20              
  Lines        2437     2519      +82     
==========================================
+ Hits         2121     2188      +67     
- Misses        316      331      +15     

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