Fix sparse operator printing

[david-pl]

Printing of sparse arrays broke at some point, e.g.

julia> destroy(FockBasis(5))
Operator(dim=6x6)
  basis: Fock(cutoff=5)sparse([1, 2, 3, 4, 5], [2, 3, 4, 5, 6], ComplexF64[1.0 + 0.0im, 1.4142135623730951 + 0.0im, 1.7320508075688772 + 0.0im, 2.0 + 0.0im, 2.23606797749979 + 0.0im], 6, 6)