(Connections) Add predicate connections

[csparker247]

M. Truszczynski suggested that it would be useful to conditionally execute a subgraph based on a predicate. This could either be an if or if-else type of situation:

// if condition
if predicate():
    signal(node)

// if-else condition
if predicate():
    signal(node0)
else:
    signal(node1)

I can easily see how this might work with the connection syntax:

OutputPort<int> op{&tgt};
InputPort<int> ip1{&src1};
InputPort<int> ip2{&src2};

// Predicate function
// Accepts argument of type T from OutputPort<T>
bool GreaterThan5(const int& i) {
    return i > 5;
}

// if condition
connect(op, ip1, GreaterThan5);

// if-else condition
// if predicate == true, post to ip1, else post to ip2
connect(op, ip1, ip2, GreaterThan5);

// Ignore the direct value, execute path if there was a node error
connect(op, ip1, [&node](int) { return node->status == Status::Error; });

What is less clear to me is how to serialize a connection like this. It's easy to serialize/deserialize the value of the predicate, but much harder to do the same for the predicate logic. This SO answer suggests registering functions in a factory, the same way we currently do our Node types. This would need a special factory that can store function references (or std::function) of different signatures. This SO answer seems like it would answer that. It's also more fun boilerplate code (#4):

smgl::RegisterFunction("GreaterThan5", GreaterThan5);
smgl::RegisterFunction("SpecificNodeError", [&node](int) { return node->status == Status::Error; });

For the lambda example, it might be useful to pass the parent node pointer to the predicate function as well.

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Migrated from GitLab educelab/smgl#7. Originally opened by @csparker247 on 2020-10-08.