`set_parameter!` for ParallelDynamicalSystem

`set_parameter!` for ParallelDynamicalSystem is buggy, but easily fixable. This is the discrete case, didn't check for continuous: ```julia using DynamicalSystems ds = Systems.henon() pds = ParallelDynamicalSystem(ds,StateSpaceSet(rand(3,2))) @which set_parameter!(pds,2,1.2) ``` Here `set_parameter!` gets dispatched to the wrong method: `set_parameter!(ds::DynamicalSystem, index, value)` which only changes the parameter of the first system from `pds.systems`. There is a method for `ParallelDiscreteTimeDynamicalSystem` ```julia set_parameter!(pdtds::PDTDS) = for ds in pdtds.systems; set_parameter!(ds, args...); end ``` but that needs to be corrected for something like ```julia set_parameter!(pdtds::PDTDS,index,value) = for ds in pdtds.systems; set_parameter!(ds, index,value); end ``` I'll make a small PR later.