[bugfix] fix double-free and invalid depth-evaluation in probabilistic gate

[corryvrequan]

• Bug description
  When we call QuantumCircuit::to_string with a quantum circuit with ProbabilisticGate in python, the kernel crashes.

• Reason
  Since the number of target qubits in ProbabilisticGate cannot be determined before execution, it returns zero as a temporal value.
  However, a function QuantumCircuit::to_string crashes when it encounters any gate of which the number of target qubits is zero.

• Update
  The behavior of QuantumCircuit::to_string is changed, and it now ignores any gate with empty target qubit list.
  I also added small test function for checking to_string with probabilistic gate.

[corryvrequan]

• Bug description

When we add ProbabilisticGate generated by functions such as DepolarizingNoise to a circuit in python, and a circuit accesses the gate, python kernel crashes.

• Reason

When we add probabilistic gate to a python in this way, python releases instances which are still used in C++. This cause that circuit accesses released gate and use it for state updates.

• Update

We redirect circuit::add_gate in python to circuit::add_gate_copy in C++,
and we make a new member function circuit::add_gate_consume only for python, which is redirected to circuit::add_gate in C++.

This makes C++-code independent of python-related pointer as default behavior.
Main drawbacks of these changes are that "add_gate" slows down since it copies gate matrix, and it requires additional memory for copying gate matrix.