[wont pass tests yet] Proposed factory / auto-subclassing Variable

[jlnav]

This won't pass all the tests as-is; it's a proposed interface but it does work with additional small adjustments to vocs.py.

    var0 = Variable(
        default_value=1.0,
        dtype=float,
    )
    assert isinstance(var0, Variable)

    var1 = Variable(
        default_value=2.0,
        domain=[0.0, 4.0],
    )
    assert isinstance(var1, ContinuousVariable)

    var2 = Variable(
        dtype=int,
        default_value=2,
        values={1, 2, 3},
    )
    assert isinstance(var2, DiscreteVariable)

    vocs = VOCS(variables={"x0": var0, "x1": var1, "x2": var2})
    assert vocs.n_variables == 3

Making the Variable base class also a factory class that produces the corresponding subclass when parameterized accordingly. Each subclass naturally validates itself. VOCS loses some validation responsibilities.

Each variable itself also has a name and dtype as discussed. dtype, default_value's type, and the values within DiscreteVariable should naturally match, and validation logic should be written for this.

This kind of factory-logic can be extended for the other objects that are currently accepted by VOCS.