Add methods to `Statevec`

[king-p3nguin]

The following methods may improve the utility of Statevec:

• [ ] .inner(): calculate the inner product of two state vectors
  • return np.abs(np.dot(sv1.flatten().conjugate(), sv2.flatten()))
• [ ] .equiv(): checks the equality of two state vectors up to the global phase
  • add atol and rtol to parameters to set precision
  • ignore_global_phase option to ignore the global phase, set True as default
• [ ] .draw(): show state vector in latex format (in jupyter notebook) or text (in command line)
  • add format to parameter ("latex" or "text")
• [ ] .to_dict(): convert Statevec to dict format