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

Patch coverage: 87.38% and project coverage change: -0.18% ⚠️

Comparison is base (d7be106) 87.03% compared to head (fb6773a) 86.85%.

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     

Files Changed	Coverage Δ
src/defs.jl	100.00% <ø> (ø)
src/moi_oracle.jl	83.92% <ø> (-0.29%)	⬇️
src/abstract_oracles.jl	80.62% <77.77%> (-5.62%)	⬇️
src/pairwise.jl	75.00% <92.30%> (+2.64%)	⬆️
src/afw.jl	95.12% <100.00%> (+0.22%)	⬆️
src/fw_algorithms.jl	85.54% <100.00%> (+0.34%)	⬆️
src/tracking.jl	60.71% <100.00%> (+4.71%)	⬆️

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