Resolve "Create SFG"

This merge will:

• add ability to "shallow-copy" graph components, which is required to implement the SFG constructor
• redesign Port/OutputPort/InputPort interface to simplify implementation and make it easier to implement operator overloading for connecting signals (only InputPort has a connect method)
• update logic between Signal constructor/set_source/set_detination and InputPort.connect/add_signal/remove_signal, etc.
• add SignalSourceProvider interface for Operation and OutputPort that forces AbstractOperation to implement a method that provides its "main" signal source if available, making it possible to pass either an OutputPort or Operation as a signal source to various operation constructors
• implement SFG constructor according to the current class diagram
• fix order of methods in various classes to match class diagram
• fix order of constructor arguments in InputPort/OutputPort to match class diagram
• move input source connection code from concrete operations to AbstractOperation to reduce boilerplate in user-defined operations
• use the public interface of base classes in derived classes instead of relying on implementation details for operations
• change param name from "coefficient" to "value" in some core operations
• add some tests for SFG construction
• fix some small typos/style errors in comments and syntax
• fix some tests relying on AssertionError being raised, rather than any Exception
• change Signal.is_connected to Signal.dangling to match class diagram
• update implementation of AbstractOperation.split() and AbstractOperation.evaluate_output()
• add implementation of evaluate() and evaluate_output() to sfg (needs testing)