Modify reshape_ and permute_ to return itself.

[hunghaoti]

I modified reshape_, permute_ functions in both UniTensor and Tensor object. So now they will return itself, just like relabel_(), instead of None as a void function.
I have test the following code and work well:

A = cytnx.UniTensor.zeros([2,3])
B = A.permute([1,0]) # A and B share tensor elements, different metadata
B[0,1] = 1.
C = A.clone().permute([1,0]) # C is independent of A
C[0,1] = 2.
print(A) # Original shape, element A[1,0] == 1.
print(B) # Permuted shape, element B[0,1] == 1.
print(C) # Permuted shape, element C[0,1] == 2.
D = A.permute_([1,0])
# D is a reference to A
print(A) # Permuted shape, element A[0,1] == 1.
print(D is A) # Output: True

A = cytnx.zeros([2,3])
A[0,1] = 1 
print(A)
D = A.permute_(1,0)
print(D)
print(D is A) # Output: True

print(D is A) gives True
This PR solve the issue #380