add `AttractorsProblem` as bridge with Attractors.jl

[oameye]

We now have HomotopyContinuationProblem <: SteadyStateProblem which together with HomotopyContinuationMethod <: SteadyStateMethod can solve for fixed points in our dynamical system (in a rotating frame) using HomotopContinuation.jl. However, a dynamical system can have more than fixed point attractors, e.g., limit cycles, periodic orbits, strange attractors.

We would like to have AttractorsProblem <: SteadyStateProblem which would define a SteadyStateProblem which together with AttractorsMethod would find the steady states using Attractors.jl.

The new method should dispatch on get_steady_states(::HarmonicEquation, ::AttractorsMethod, ...) and the solutions should be saved in the Result struct.