Add states labels to Channels and SequenceSamples

[a-corni]

• [x] I am associating states to the bases addressed by Channels
• [x] such that they can be combined to make addressed states in Device and Sequence
• [x] and they unify the handling of these quantities in hamiltonian.py Conventions.rst is also modified to take into account these labels, but I am not sure it actually improves the understanding of the conventions since it introduces a different label for XY.

[a-corni]

Is this good enough ? Otherwise I am ready for review