embed may not embed for a CompositeBasis of one element

[amilsted]

When embed builds the result by performing tensor products, it will not produce an operator on a CompositeBasis if there is only one element in the input composite basis. This is unexpected.

[amilsted]

Relatedly, should the following even be allowed to exist?

• CompositeBasis with a single element
• Nested CompositeBasis

Currently, both can be generated.

[AmitRotem]

Just stumbled upon this issue while sparsing a LazyTensor of a single element CompositeBasis.
So I find it useful and think they should exist.

b=NLevelBasis(2)
cb=CompositeBasis(b)
op=randoperator(b)
lop = LazyTensor(cb, 1, op)
basis(op) == basis(sparse(lop)) # false
basis(op) == basis(dense( lop)) # false

Also, currently embed has inconsistent behavior with this basis

basis(embed(cb,      1 , op )) == cb # true
basis(embed(cb,     [1],[op])) == cb # false
basis(embed(cb,Dict([1]=>op))) == cb # false