multivector bloch_sphere should raise an error

[1ucian0]

When introducing phase kickback to the general public I realised that multivector bloch sphere has no labels. That can be confusing when you are not very familiar with the polar basis.

state = QuantumCircuit(2)
state.h(0)
state.x(1)
state.h(1)

bloch_sphere(state.statevector()))

This is particularly necessary when the vectors overlap:

state = QuantumCircuit(2)
state.h(0)
state.x(1)
state.h(1)
state.cx(0, 1)

bloch_sphere(state.statevector())

[nonhermitian]

Seeing this now. It should probably raise an error for a multi-qubit statevector as that is definitely not something a Bloch sphere can handle. The typical use case for displaying multiple vectors is for when you want to look at the evolution of a single-qubit state over some time or series of steps.