Support MOI.FEASIBILITY_SENSE

[rschwarz]

Currently (after merging #76), setting MOI.FEASIBILITY_SENSE is a no-op.
We coud clear the objective coefficients of all variables, but then they could not be recovered when the objective sense is set to maximize or minimize.

To support this properly, we would have to store both the sense and the objective coefficients separately in the MOI wrapper (SCIP.Optimizer).

[rschwarz]

Together with this change, we could also support SingleVariable objective functions (and remember that it was this function type?).

[rschwarz]

Another related missing feature is the attribute MOI.ObjectiveFunctionType. This is used, e.g., by JuMP to show the model in the REPL, so it currently errors with a direct SCIP.Optimizer.