a bunch of methods acting on abstract types assume internal layout for these types

[Krastanov]

E.g.:

• QuantumInterface.jl/src/julia_base.jl

  Line 15 in d8973a7

  basis(a::StateVector) = a.basis

• QuantumInterface.jl/src/julia_base.jl

  Line 34 in d8973a7

  basis(a::AbstractOperator) = (check_samebases(a); a.basis_l)

Long term, a better design would be to move these to QuantumOpticsBase where they will act on concrete (not abstract) types (this is where they came from). Methods acting on abstract types should use only public APIs, not assumptions about internal layout, because the internal layout might be different in other libraries extending this functionality. E.g. QuantumClifford.PauliChannel superoperators do not have basis_l slots, unlike QuantumOptics superoperators.

[akirakyle]

Reopening since there's still access of .basis in many places which should be fixed in conjunction with #40.