Add state control functions

[corryvrequan]

I've added three functions to control quantum state objects.

1. tensor_product

We can generate a tensor product of two quantum states.

Usage

from qulacs.state import tensor_product
from qulacs import QuantumState

qs1 = QuantumState(3)
qs2 = QuantumState(5)
qs3 = tensor_product(qs1, qs2)
# qs3 is 8-qubit state
# np.kron(qs1.get_vector(), qs2.get_vector()) = tensor_product(qs1, qs2).get_vector()

2. permutate_qubit

We can permutate the order of qubits in a quantum state.

Usage

from qulacs.state import permutate_qubit
from qulacs import QuantumState

qs1 = QuantumState(3)
qs2 = permutate_qubit(qs1, [1,0,2])
# qs1's [0,1,2,3,4,5,6,7]-th elements are ordered as [0,2,1,3,4,6,5,7]

3. drop_qubit

We can drop a set of qubits from a quantum state with projecting to a given subspace.

Usage

from qulacs.state import drop_qubit
from qulacs import QuantumState

qs1 = QuantumState(3)
drop_index = [2,1]
projection = [0,1]
qs2 = drop_qubit(qs1, drop_index, projection)
# 2nd and 1st qubits are removed, and projected to a subspace of |01*> (rightmost is 0-th qubit).
# Thus, qs2[0] = qs1[2], qs2[1] = qs1[3]

[corryvrequan]

Since this feature works correctly and seems correct as far as I tested. So let me merge it and fix bugs if there are.