check norm is real or not

[yomichi]

tenes assumed that the norm is real, but a bad simulation condition (e.g., too small of the bond dimension of CTM) causes the norm to be complex.

After this PR:

• In evaluating expectation values of observable, the denominator is now <Ψ|Ψ> itself, not real(<Ψ|Ψ>).
• tenes checks whether the norm is real or complex
  • if the norm is complex, tenes warns.
  • the threshold is 1e-6.
• The calculated norm is outputted as an operator with the index of -1 in onesite_obs.dat and twosite_obs.dat.