Add functionality to select the solver to be used

[danielolsen]

Currently, we are hard-coded for Gurobi. We would like to support at least one open-source solver (though performance will be slow), and potentially other commercial solvers as well.

[danielolsen]

I think there are a couple of different ways we could go about this. If we expect that the user will be interacting with the Julia code directly, then we can have an optional parameter to REISE.run_scenario that takes a MathOptInterfact.ModelLike to create a model with (see https://jump.dev/JuMP.jl/dev/reference/models/#JuMP.direct_model).

If we want the different solvers to be accessible from the python side, this becomes more difficult, because we will need to instantiate the ModelLike objects (e.g. a CPLEX solver) within the REISE.jl code directly, which means that we will need to have e.g. CPLEX.jl already installed. It seems that how to support 'optional dependencies' is still an active discussion, see JuliaLang/Pkg.jl#1285.