Add get_block_unitensor() and get_block_unitensor_()

[pcchen]

We need to add get_block_unitensor() and get_block_unitensor_() which return a UniTensor instead of a Tensor.

[kaihsin]

Actually, I think there is a better way to do this.

If we just make Tensor has a method Tensor.unitensor(labels)

[kaihsin]

So the idea is we can do

UT.get_block().unitensor(labels) 

In the user guide, we can say:

``get_block() return the data of a block (as a Tensor, see Appendix.XXX). To get the block as a unitensor, simply chaining with .unitensor(labels) as following:"

UT.get_block().unitensor(labels) 

[kaihsin]

and for the generator, one can do

cytnx.zeros(100).unitensor(labels)