Fix return type of SimplexIterations

[odow]

julia> begin
           model = read_from_file("benchmark/stroemer.mps.gz")
           set_optimizer(model, Gurobi.Optimizer)
           set_silent(model)
           optimize!(model)
           solution_summary(model)
       end
* Solver : Gurobi

* Status
  Result count       : 1
  Termination status : OPTIMAL
  Message from the solver:
  "Model was solved to optimality (subject to tolerances), and an optimal solution is available."

* Candidate solution (result #1)
  Primal status      : FEASIBLE_POINT
  Dual status        : FEASIBLE_POINT
  Objective value    : 2.77048e+05
  Objective bound    : 2.77048e+05
  Dual objective value : 2.77048e+05

* Work counters
  Solve time (sec)   : 2.98400e-02
  Barrier iterations : 0
  Node count         : 0


julia> simplex_iterations(model)
ERROR: TypeError: in typeassert, expected Int64, got a value of type Float64
Stacktrace:
 [1] _get_model_attribute(model::MathOptInterface.Utilities.CachingOptimizer{…}, attr::MathOptInterface.SimplexIterations)
   @ MathOptInterface.Utilities ~/.julia/packages/MathOptInterface/1fRdT/src/Utilities/cachingoptimizer.jl:865
 [2] get(model::MathOptInterface.Utilities.CachingOptimizer{…}, attr::MathOptInterface.SimplexIterations)
   @ MathOptInterface.Utilities ~/.julia/packages/MathOptInterface/1fRdT/src/Utilities/cachingoptimizer.jl:900
 [3] _moi_get_result(model::MathOptInterface.Utilities.CachingOptimizer{…}, args::MathOptInterface.SimplexIterations)
   @ JuMP ~/.julia/packages/JuMP/6RAQ9/src/optimizer_interface.jl:1053
 [4] get(model::Model, attr::MathOptInterface.SimplexIterations)
   @ JuMP ~/.julia/packages/JuMP/6RAQ9/src/optimizer_interface.jl:1073
 [5] simplex_iterations(model::Model)
   @ JuMP ~/.julia/packages/JuMP/6RAQ9/src/optimizer_interface.jl:927
 [6] top-level scope
   @ REPL[12]:1
Some type information was truncated. Use `show(err)` to see complete types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.51%. Comparing base (b8cea15) to head (855035d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #573   +/-   ##
=======================================
  Coverage   91.51%   91.51%           
=======================================
  Files           5        5           
  Lines        2450     2450           
=======================================
  Hits         2242     2242           
  Misses        208      208           

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