add SZeroOperator, SZeroBra, SZeroKet structs

[apkille]

julia> A = SOperator(:A, SpinBasis(1//2)); b = SBra(:b, SpinBasis(1//2)); k = SKet(:k, SpinBasis(1//2));

julia> @show 0*A 0*b 0*k;
0A = 0A
0 * b = 0⟨b|
0k = 0|k⟩