Move express and representation types to QuantumInterface

[apkille]

Right now, I'm working on translating phase space representations of Gaussian quantum objects in Gabs.jl to Fock space representations in QuantumOptics.jl. Should we limit express to one way symbolic-to-numerical translations, or could we use it as a general function that translates between any two packages that depend on QuantumInterface.jl? To me, it seems pretty clean to implement something like express(::GaussianState, QuantumOpticsRepr(cutoff = 10)) and return a QuantumOptics Ket object with FockBasis(10).

[Krastanov]

I am in favor of this.

QuantumClifford already has an extension that converts some objects to QuantumOptics objects, but those are unambiguous and do not need configuration like cutoff=10. For the more general objects you are describing, this seems like the appropriate way to proceed.