Unable to create `S_exchange` directly as real TensorMap

[Yue-Zhengyuan]

MWE:

using MPSKitModels
S_exchange(Float64)

Output:

ERROR: InexactError: Float64(0.0 - 0.5im)
Stacktrace:
  [1] Real
    @ ./complex.jl:44 [inlined]
  [2] convert
    @ ./number.jl:7 [inlined]
  [3] setindex!(::Matrix{Float64}, ::ComplexF64, ::Int64, ::Int64)
    @ Base ./array.jl:983
  [4] spinmatrices(s::Rational{Int64}, elt::Type)
    @ MPSKitModels ~/.julia/packages/MPSKitModels/RcUP6/src/operators/spinoperators.jl:31
  [5] #S_x#42
    @ ~/.julia/packages/MPSKitModels/RcUP6/src/operators/spinoperators.jl:61 [inlined]
  [6] S_x
    @ ~/.julia/packages/MPSKitModels/RcUP6/src/operators/spinoperators.jl:60 [inlined]
  [7] S_xx(elt::Type{Float64}, ::Type{TensorKit.Trivial}; spin::Rational{Int64})
    @ MPSKitModels ~/.julia/packages/MPSKitModels/RcUP6/src/operators/spinoperators.jl:382
  [8] S_xx
    @ ~/.julia/packages/MPSKitModels/RcUP6/src/operators/spinoperators.jl:381 [inlined]
  [9] S_exchange(elt::Type{Float64}, ::Type{TensorKit.Trivial}; spin::Rational{Int64})
    @ MPSKitModels ~/.julia/packages/MPSKitModels/RcUP6/src/operators/spinoperators.jl:422
 [10] S_exchange(elt::Type{Float64}, ::Type{TensorKit.Trivial})
    @ MPSKitModels ~/.julia/packages/MPSKitModels/RcUP6/src/operators/spinoperators.jl:421
 [11] S_exchange(elt::Type{Float64}; kwargs::@Kwargs{})
    @ MPSKitModels ~/.julia/packages/MPSKitModels/RcUP6/src/operators/spinoperators.jl:416
 [12] top-level scope
    @ REPL[3]:1

It appears that the construction uses $S^y$, so I cannot directly set eltype as a real type. Although I can circumvent the issue by real(S_exchange()), it may still be better to be able to directly create it as a real operator. A possible solution (at least for spin-1/2) is to use $(1/2)(S^+_1 S^-_2 + h.c.) + S^z_1 S^z_2$ instead of $S^x_1 S^x_2 + S^y_1 S^y_2 + S^z_1 S^z_2$.

[lkdvos]

That still poses issues, since S_plus is defined as S_x + im S_y. (As an aside, this is probably why these had the opposite conventions from the raising and lowering operators, but oh well...)
I'll try and fix it just by adding a real at the end if necessary.