Suggestion: refactor __repr__ methods to output shape tuples instead of MATLAB-like size strings

[dmdunla]

Suggestion: Change class instance output (via __repr__) to output shapes as tuples

Background: When making changes for #145 , I added a test of __repr__ for empty ktensor instances and saw an inconsistency of how the shape attribute is output in this case.

In MATLAB, here is the output string (for a ktensor for example):

B is a ktensor of size 3 x 4 x 5

Here is the proposed corresponding output string in pyttb:

ktensor of shape (3, 4, 5)

Several motivators for this suggestion:

1. Output reflects that the shape attribute is a tuple.
2. Consistency with numpy-like shape attributes:
   • Empty: ktensor of shape () (rather that ktensor of shape 0, which may confuse users)
   • 1-way: ktensor of shape (2,) (rather than ktensor of shape 2, which may confuse users)
   • d-way: ktensor of shape (3, 4, 5)

I made the changes in a current PR (#182), but would like input before merging the changes.

[dmdunla]

I merged the changes suggested above to not hold up the ktensor linting PR. I can revert if there is strong reason to stay with the original MATLAB-like shape description in __repr__.