Slicing should have correct rowrank set

[manuschneider]

At the moment, slicing a UniTensor returns a UniTensor with rowrank=0.
I would suggest that all remaining indices in the rowspace/columnspace should remain in the corresponding space.
Example:

uT=cytnx.UniTensor.zeros([2,3,4], name="uT", labels=["a", "b", "c"])
uT.set_rowrank_(2)
slice1 = uT[:,:,0:2]
slice2 = uT[0,:,0:2]
print(uT.rowrank())     # 2
print(slice1.rowrank()) # 0, I would suggest it should be 2
print(slice2.rowrank()) # 0, I would suggest it should be 1 because one dimension was removed