ROI_applicable_solvers takes as argument an optimization problem (object of class 'OP') and returns a vector giving the applicable solver. The set of applicable solver is restricted on the available solvers, which means if solver "A" and "B"
would be applicable but a ROI.plugin is only installed for solver "A" only solver "A" would be listed as applicable solver.
ROI_applicable_solvers(op)
Arguments
op: an ROI-object of type 'OP'.
Returns
An character vector giving the applicable solver, for a certain optimization problem.