Modify defaults settings for Gurobi

[filippopecci]

GenX currently sets these default settings for Gurobi:

default_settings = Dict("Feasib_Tol" => 1e-6,
        "Optimal_Tol" => 1e-4,
        "Pre_Solve" => -1,
        "AggFill" => -1,
        "PreDual" => -1,
        "TimeLimit" => Inf,
        "MIPGap" => 1e-3,
        "Crossover" => -1,
        "Method" => -1,
        "BarConvTol" => 1e-8,
        "NumericFocus" => 0,
        "OutputFlag" => 1)

Given that GenX models have large sizes in most cases (and are quite sparse) we should set as default method the Barrier algorithm. Also, when using the Barrier method, we should not set feasibility and optimality tolerances, and just use BarConvTol, which applies to both feasibility and optimality - as explained here (slide 38). So I suggest we change the defaults to:

default_settings = Dict("MIPGap" => 1e-3,
        "Crossover" => 0,
        "Method" => 2,
        "BarConvTol" => 1e-6,
        "OutputFlag" => 1)

The user is always free to change more options when setting up the gurobi_settings.yml file, but this should be our basic configuration.