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)'OP'.