Support for cyclical graphs about mdf

[pgleeson]

Potential extension for cyclical graphs: add parameter cycle_default_value - the default value only to use when it's in a cycle, not standalone.

[Devadeut]

@pgleeson in addition to this:
snippet from src/modeci_mdf/execution_engine.py

---

def init(self, node: Node, verbose: Optional[bool] = False):
self.verbose = verbose
self.node = node
self.evaluable_inputs = {}
self.evaluable_parameters = OrderedDict()
self.evaluable_functions = OrderedDict()
self.evaluable_outputs = {}

---

Can using OrderedDict() be the reason for the existing constraint?

[pgleeson]

Not really. It's basically that one of the nodes in a cycle (e.g. A->B->C->A) needs to be flagged as "this is the one to start evaluating the nodes at". We will do this by stating that an input node has a default value, and that this can be used to start the evaluation, e.g. if A has a default value, you don't have the problem that you first need to evaluate the output of C, but before that the output of B, but before that A...