More test cases needed for the ITensorNetworks module

[LinjianMa]

Currently the ITensorNetworks module only has 3 tests, two for the projectors and 1 for the model.

We need to add more unit tests, for both testing the function robustness and for demonstrating the API/purpose of each function.

[mtfishman]

Agreed it could use more testing.

A lot of the functionality was exploratory as I was trying out different designs and contraction strategies. It has been cleaned up, reorganized and simplified in #16 so should be easier to understand and test now.