Merge API for UniTensor.Contract() and UniTensor.Contracts()

[pcchen]

Right now, we have

• UniTensor.Contract() which contract two UniTensors.
• UniTensor.Contract() which contract multiple UniTensors. This might be confusing to users. We should merge these APIs.

We should first think about what the new API should look like.

[pcchen]

This is to confirm that all following codes work properly. Please merge it into the master branch.

cytnx.Contract(A, B)
cytnx.Contract([A, B])
cytnx.Contract([A, B, C])

Question: Do we want to depreciate cytnx.Contract(A, B) and only support `cytnx.Contract([A, B])' in the future?