Singleton column stuffing

[fwesselm]

• This PR adds the presolve reduction singleton column stuffing as described by Gamrath et al., Progress in presolving for mixed integer programming. Math. Prog. Comp. 7, 367–398 (2015).
• My tests confirmed that the reduction is not very effective on MIPLIB etc.
• However, the reduction is cheap and, as stated in the paper, may be helpful on some practical instances (e.g., from supply chain management).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.08%. Comparing base (978a958) to head (113e9d1).
⚠️ Report is 35 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #2623      +/-   ##
==========================================
+ Coverage   81.06%   81.08%   +0.01%     
==========================================
  Files         347      347              
  Lines       85228    85309      +81     
==========================================
+ Hits        69092    69173      +81     
  Misses      16136    16136              

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