Issues with get_subgraph method of CouplingGraph

[alonkukl]

I see a few issues with the method get_subgraph of CouplingGraph.

Consider the following code, where we create a 5-qubit all-to-all, and then ask for a specific subgraph :

cg = CouplingGraph.all_to_all(5)
sub_g = cg.get_subgraph((1,3,4))

The issues I see are:

1. The default behavior of the function is to rename the qubits from 0 to j (where j Is the size of the subgraph). This is not documented and is not the default behavior I would expect.
2. When trying to provide the function with a renaming dict, the code crashes:

cg.get_subgraph((1,3,4), {i:i for i in range(5)})

The crash:
ValueError: Edges between invalid qudits or num_qudits too small.

3. The class also has a method named 'get_induced_subgraph', that doesn't change the name of the qubits, but returns a list rather than a CouplingGraph. The issues here is why is there a code duplication? Why is the default different?