Refactor the Gurobi.jl requirement

[danielolsen]

As a follow-up to #99, now that other solvers can be used, it would be nice to remove the requirement that Gurobi.jl be installed as a part of REISE.jl. It will need to be installed at some point to be able to implement the shared-environment feature that's currently a part of run_scenario.

The first design idea that comes to mind is a 'wrapper' module, named something like REISE_Gurobi, that handles all Gurobi-specific code, with changes to REISE.jl to remove the Gurobi-specific code while still allowing the same functionality, and pyreisejl will import REISE_Gurobi instead of REISE directly. I'm open to other suggestions however.

[danielolsen]

Instead of a wrapper module, there may be another design pattern that we want to use, leveraging Requires.jl. This would let us include code within the src/REISE.jl file, as part of the REISE module, that only executes if the Gurobi package has already been loaded (which could be done within pyreise.call.launch_scenario). That way, we can avoid calling import Gurobi within the main body of src/REISE.jl.