[WIP] Add subsolver reducer

[ArrogantGao]

Added a reducer SubsolverReducer, when the problem is small enough it will use a subsolver such as xiao2013 or ip to solve the problem directly.

[ArrogantGao]

I am working on testing this strategy

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/OptimalBranchingMIS/src/reducer.jl	50.00%	9 Missing ⚠️

Files with missing lines	Coverage Δ
lib/OptimalBranchingMIS/src/reducer.jl	86.50% <50.00%> (-6.16%)	⬇️

... and 3 files with indirect coverage changes

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