Handle single basis in embed

This commit addresses the issue where the `embed` function fails when it receives a single basis instead of a composite basis as input.

src/embed_permute.jl

@@ -46,6 +46,19 @@ embed(basis_l::CompositeBasis, basis_r::CompositeBasis, index::Integer, op::Abst
 embed(basis::CompositeBasis, indices, operators::Vector{T}) where {T<:AbstractOperator} = embed(basis, basis, indices, operators)
 embed(basis::CompositeBasis, indices, op::AbstractOperator) = embed(basis, basis, indices, op)
 
+"""
+ embed(basis::Basis, indices, ops::AbstractOperator)
+ 
+Special case of the `embed` to handle embedding an operator `ops` into a single
+basis.
+"""
+function embed(basis::Basis, indices, ops::AbstractOperator)
+ if indices == 1 || indices == (1,) || indices == [1]
+ return ops
+ else
+ throw(ArgumentError("Invalid indices for single basis."))
+ end
+end
 
 """
  embed(basis1[, basis2], indices::Vector, operators::Vector)