adding an `nsubsystems` to QuantumInterface

[Krastanov]

@david-pl , @amilsted , @ChristophHotter do you have anything against publicly introducing an nsubystems function, shared with QuatnumClifford and others, by declaring it in QuantumInterface.

My last PR added such a function for internal consumption in apply!, which makes it much easier to detect when embed is needed. Here is the current implementation:

nsubsystems(s::Ket) = nsubsystems(s.basis)
function nsubsystems(s::Operator)
    s.basis_l == s.basis_r || throw(ArgumentError("`nsubsystem(::Operator)` is well defined only if the left and right bases are the same"))
    nsubsystems(s.basis_l)
end
nsubsystems(b::CompositeBasis) = length(b.bases)
nsubsystems(b::Basis) = 1

[david-pl]

@Krastanov do it!

[Krastanov]

done by #146