Merge API for UniTensor.relabel() and UniTensor.relabels() and their inplace version

[pcchen]

We should

• Merge API for UniTensor.relabel() and UniTensor.relabels().
• Merge API for UniTensor.relabel_() and UniTensor.relabels_().

We should first discuss what the new API the look like.

[pcchen]

This is to confirm that following codes work properly. Please merge this into master.

A = cytnx.UniTensor.ones([2,2]).relabel(["a","b"]).set_name("A")
A = cytnx.UniTensor.ones([2]).relabel(["a"]).set_name("A")
uT.relabel_(["b","c"], ["j","k"])
uT.relabel_(["d"], ["l"])
uT.relabel_("d", "l")
uT.relabel_(old_label="a", new_label="i")

[pcchen]

Do we want to change "labels" in keyword argument and method to "label"? For example

A = cytnx.UniTensor.ones([8,2,8], labels=["v1","phy","v2"])
or
A = cytnx.UniTensor.ones([8,2,8], label=["v1","phy","v2"])

[pcchen]

I just notice that we also have UniTensor.set_label() and UniTensor.set_labels().

• We should also merge these two.
• When should one use this instead of relabel?

[pcchen]

For example, we have UniTensor.labels(). Do we want to change this to UniTensor.labels()?