Introduction of addition and multiplication to the Hamiltonian

[Jacomichi]

Change

• I have introduced addition and multiplication to the Hamiltonian.
• The return values of X(idx), Y(idx), and Z(idx) have been changed to the Hamiltonian class.
• Fixed a bug in the conversion to qiskit Hamiltonian.
• I have introduced Pauli.I.

Example

import qamomile.core.operator as qm_o

x0 = qm_o.X(0)
h1 = 2.0 * x0 + 1.0
h = h1 * h1 # 4x0 + 5.0
expected_h = qm_o.Hamiltonian()
expected_h.constant += 5.0
expected_h.add_term((qm_o.PauliOperator(qm_o.Pauli.X, 0),), 4.0)

assert h == expected_h