Formally let StateSpaceSet subtype AbstractVector and implement `Array` interface

[kahaaga]

Copying @Datseris's points in the thread here: JuliaDynamics/ComplexityMeasures.jl#418 (comment)

StateSpaceSet needs to be reworked to subtype AbstractVector, and to formally implement Julia's vector (array) interface. Broadcasting then will "just work", and the whole vec wrapping will become obsolete. I simply haven't gotten around doing that yet, because it will create the breaking change that now size(ssset) will be (length(ssset), ) instead of (length(ssset), dimension(ssset)) that it is now, and I haven't checked wherther any downstream package utilizes this assumption. None should, because dimension(ssset) should always be used for hte number of columns, but of course some code in DynamicalSystems.jl is about 5-6 years old so you never know.

The best way forwards in my opinion is to fix the source: make a v2 of SSSets.jl that makes SSSet subtype vector, and also make it so that the element type of the inner vectors is parameterized, so that we can also solve #18 at the same time.