Handle single basis in embed

[a-eghrari]

This commit addresses the issue where the embed function fails when it receives a single basis instead of a composite basis as input. #159

[Krastanov]

I only now noticed I have not enabled the tests for your PR. Apologies for that, Github does not run tests automatically on first PR as a security precaution.

Running the tests reveals that it would be best to make this change in the foundational package QuantumInterface.jl, otherwise we have a couple of issues:

• This change will not be picked up by other libraries that might benefit from it
• It causes type piracy given that Basis is not owned by this package, but it is owned by QuantumInterface.jl

Could you consider making the contribution to that other package?

Also, to avoid ambiguity, it should probably be defined for embed(basis::Basis, indices, ops::AbstractOperator) (i.e. there should be a type assert on the last argument).

Thank you again for the contribution. Really sorry for making you wait so long for a review.

[Krastanov]

I see you have made a PR to QuantumInterface at qojulia/QuantumInterface.jl#24
Thank you! I will close this PR here and proceed there.