Add an IPM warning when symbol and attribute isn't used in edges

[ewilli1]

Suggest a warning message for users when an IPM has attributes and symbols that are not used within the edges. Joe and I discussed this in a meeting. The code below should show an example where gamma is an attribute with the y symbol but it's not used in the edges so having a warning message or alert of some kind in a scenario like this is the idea.

def construct_ipm():
    symbols = create_symbols(
        compartments=[
            compartment('S'),
            compartment('I'),
            compartment('R'),
            compartment('H'),
        ],
        attributes=[
            AttributeDef('beta', type=float, shape=Shapes.TxN,
                         comment='infectivity'),
            AttributeDef('gamma', type=float, shape=Shapes.TxN,
                         comment='infectivity'),
            AttributeDef('xi', type=float, shape=Shapes.TxN,
                         comment='progression from recovered to susceptible'),
            AttributeDef('h_rate', type=float, shape=Shapes.TxN,
                         comment='progression from infected to hospitalized'),
            AttributeDef('h_recovery', type=float, shape=Shapes.TxN,
                         comment='progression from hospitalized to recovered'),
            AttributeDef('h_prob', type=float, shape=Shapes.TxN,
                         comment='probablity for hospital versus recovered'),
        ])

    [S, I, R, H] = symbols.compartment_symbols
    [β, y, ξ, h_rate, h_rec, h_prob] = symbols.attribute_symbols

    N = Max(1, S + I + R)

    return create_model(
        symbols=symbols,
        transitions=[
            edge(S, I, rate=β * S * I / N),
            fork(
                edge(I, H, rate=h_rate * I * h_prob),
                edge(I, R, rate=h_rate * I * (1 - h_prob)),
            ),
            edge(H, R, rate=h_rec * H),
            edge(R, S, rate=ξ * R)
        ])
SIRH_ipm = construct_ipm()  # store hospitalization ipm