Fix Bug: General SAT to Independent Set

[SciCodePhy]

Conclusion: Previous update of "SAT => Independent Set" (#69 ) is problematic. For single input, it is impossible to avoid returning multiple degenerate solutions. Therefore, I go back to the initial method to construct the independent set graph (#65 ), which is consistent with the references.

Achieved:

• Choose the correct reduction;
• Give warnings when return multiple degenerate solutions for single input;
• Modify the way to construct the set of solutions;
• Add more tests for single input and multiple inputs;

Reason and Example
There are two cases when the single input returns multiple degenerate solutions.
Assuming that the literals are x, y and z:

• Case01 (satisfiable SAT): Independent Set might give solutions with repeated literals such as "x=0, y=1, x=0, y=1". This case means that z can take arbitary value (truse or false, 1 or 0).
• Case02 (unsatisfiable SAT): Independent Set will give solutions which don't cover all the literals, such as "x=0, y=1", because whenever z is true or false the whole SAT is unsatisfiable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.76%. Comparing base (edb2cdb) to head (b279101).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   90.55%   93.76%   +3.21%     
==========================================
  Files          28       28              
  Lines        1217     1203      -14     
==========================================
+ Hits         1102     1128      +26     
+ Misses        115       75      -40     

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[GiggleLiu]

Looks good to me.