Easier and future proof way to tell is a vertex is constant.

[chris-major-improbable]

While analysing a network of Vertices it's not obvious which have constant values.
For example I've got the following code:

    private static boolean isConstant(DoubleVertex vertex) {
        if ( vertex instanceof ConstantDoubleVertex ){
            return true;
        } else if ( vertex instanceof DoubleBinaryOpVertex ){
            DoubleBinaryOpVertex dbv = (DoubleBinaryOpVertex)vertex;
            return isConstant(dbv.getLeft()) && isConstant(dbv.getRight());
        }
        return false;
    }

I think this would be useful as a core part of keanu so it's maintained

Alternatives
The main issue with this would be "custom" vertices from other sources. It may be better to provide an interface that let's code understand a vertex isn't going to change.

Additional context
Eventually this might even allow some automatic optimisations (e.g. cache values if they can never change).

[migwellian]

This sounds like a good idea to me. I think we can make it extensible to users without the need for a new marker interface. Does this logic make sense:

A vertex is constant if:

1. It's not Probabilistic (this is now an interface)
2. AND if it has parents, they are all constant