Add qubit state generation, truncation and expansion.

[BoxiLi]

Changes

• Add truncation and expansion with qubit state
  • Truncate multi-level state into a qubit state discarding the non-computational basis.
  • Expand a qubit state to a multi-level state, filled with zeros.
• Support a more general way of qubit state definition:
  • use a list or a string for zero/one/plus/minus state
  • use a list of integers for defining arbitrary disentangled quantum states (up to a global phase).
  • The parameter N is removed. It does not make sense to expand the input with zero states under the new definition. The number of the qubits should just be the same as the length of the input.

Question
Should the name be qubit_states or qubit_state?

[BoxiLi]

@purva-thakre If you have time, would you like to take a review on this PR? Maybe the qubit_states function is interesting to you when you write tests etc further for the decomposition. You could try the functions a bit and ask for explanations/changes on whatever is not clear or you think is wrong. No need to push it if you are busy :)

[purva-thakre]

@BoxiLi Sure, I could take a look (might take me a couple of days).

[BoxiLi]

Suggested change

	"""Find the forresponding indices for a given qu"""
	"""Find all the qubit indices for a given dimension, e.g. ``[3, 4]``"""