[WIP] fix config feature add

[Fanerst]

Add the feature that allows fixing part of the configuration in order to find a solution with smaller contraction complexity.

[GiggleLiu]

Thanks for the pull request! It is really a useful feature to have.

I added a few comments, there are also some general aspects that this PR can be improved. The first one is adding a test, like try fixing some configurations and enumerate the solution space, then see if it works as expected. This is a good place to have the test: https://github.com/QuEraComputing/GenericTensorNetworks.jl/blob/master/test/networks/MaxCut.jl .

It would be nice if you can add a similar feature to other graph problems, which is not hard, I can help with that.

The priority of the changes is

correctness > performance > the rest (e.g. naming)

One you have a test and the CI passes, I can approve this PR.
Thanks again for your effort!

[codecov-commenter]

Codecov Report

Merging #40 (506f460) into master (2ebb1bf) will increase coverage by 0.09%.
The diff coverage is 96.49%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   93.64%   93.73%   +0.09%     
==========================================
  Files          24       24              
  Lines        1432     1453      +21     
==========================================
+ Hits         1341     1362      +21     
  Misses         91       91              

Impacted Files	Coverage Δ
src/GenericTensorNetworks.jl	66.66% <ø> (ø)
src/networks/PaintShop.jl	77.77% <66.66%> (+0.63%)	⬆️
src/networks/Coloring.jl	96.00% <100.00%> (+0.16%)	⬆️
src/networks/DominatingSet.jl	95.45% <100.00%> (+0.21%)	⬆️
src/networks/IndependentSet.jl	96.77% <100.00%> (+0.10%)	⬆️
src/networks/Matching.jl	90.69% <100.00%> (+0.22%)	⬆️
src/networks/MaxCut.jl	95.45% <100.00%> (+0.45%)	⬆️
src/networks/MaximalIS.jl	95.45% <100.00%> (+0.21%)	⬆️
src/networks/Satisfiability.jl	97.95% <100.00%> (+0.04%)	⬆️
src/networks/SetCovering.jl	97.05% <100.00%> (+0.08%)	⬆️
... and 2 more

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[GiggleLiu]

Cheers!