[BUG] Multiple composition

[iincer]

Describe the bug
Multiple composition has incorrect top-level IO.

To Reproduce
This is the current test:

def test_multiple_composition() -> None:
    c1 = PolyhedralContract.from_string(
        assumptions=["x <= 1"], guarantees=["y <= 0"], input_vars=["x"], output_vars=["y"]
    )
    c2 = PolyhedralContract.from_string(
        assumptions=["y <= 0"], guarantees=["z <= 0"], input_vars=["y"], output_vars=["z"]
    )
    c3 = PolyhedralContract.from_string(
        assumptions=["y <= 0"], guarantees=["q <= 0"], input_vars=["y"], output_vars=["q"]
    )
    c4 = PolyhedralContract.from_string(
        assumptions=["q <= 0"], guarantees=["v <= 0"], input_vars=["q"], output_vars=["v"]
    )

    contract = compose_multiple_contracts([c1, c2, c3, c4])

    assert str(contract.a.to_str_list()[0]) == "y <= 0.0"

The assertion shown implies that y is a top-level input of the composition, which is wrong, as the only top-level input should be x. When we print the composed contract, we get

InVars: [y]
OutVars:[z]
A: [       
  y <= 0.0 
]
G: [       
  z <= 0.0 
]

Expected behavior
x should be a top-level input to the composed contract. I am puzzled as to where x went in this composition.
Note that the contract contract is exactly the same as c2.
The implementation and test need to be fixed. Please add more tests to make sure the implementation is right.

[iincer]

@pierg, please note there is a branch associated with this issue. Please create a PR when ready.