Added kImplicitInteger to definition of HighsVarType for highspy

[jajhall]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.75%. Comparing base (3ff15e4) to head (60df046).
Report is 3 commits behind head on latest.

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #2170   +/-   ##
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  Coverage   78.75%   78.75%           
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  Files         345      345           
  Lines       83165    83165           
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  Hits        65498    65498           
  Misses      17667    17667           

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[odow]

Question: when should we use kImplicitInteger instead of kInteger?

[jajhall]

Users shouldn't use kImplicitInteger when defining models: it's an artefact of presolve.

It's a continuous variable that, by implication, must take an integer value. For example, if it is the only continuous variable in an equation with integer coefficients and RHS. The fact that it can only take integer values is exploited in the MIP solver. For example, if a domain placed it in an interval interior to consecutive integer, then the corresponding node would be infeasible.