Layout is bijective dict only, transpile accomodates list and dict

[ajavadia]

Summary

Fixes #2032

1. The Layout class now only has a constructor from dict, and this dict must be bijective.

2. For user convenience, transpile() accepts this dict in addition to a list (so the following scenarios are supported):

a) running a single qubit experiment on qubit 19
i. virtual2physical dict
{qr[0]: 19}
ii. physical2virtual dict
{19: qr[0]}
iii. v2p list
[None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, qr[0]]
iv. p2v list
[19]

b) Bell pair on 2 qubits out of 5
i. v2p dict
{qr[0]: 0, qr[1]: 3}
ii. p2v dict
{0: qr[0], 3: qr[1]}
iii. v2p list
[qr[0], None, None, qr[1], None]
iv. p2v list
[0, 3]

3. Previously non-bijectivity was (ab)used for ancilla allocation: first create a bunch of None entries in the Layout, then allocate ancilla for those. I have changed the two passes that did this.
   We now have:

• FullAncillaAllocation, a pass that examines a DAG, a layout and a coupling_map, to see whether there are free physical qubits. If so, it allocates all those physical qubits as ancillas, AND updates the layout with new (physical, virtual) pairs.
• ExtendWithAncilla, a pass that examines a DAG and a layout, and if the layout contains entries that are not in the DAG, updates the dag accordingly (adds new virtual qubits to the circuit, to act as ancilla).