Resolve "Generate PG from SFG"

Adds:

• A function that returns a nested list which describes the precedence of all of the output ports in an sfg. Output ports with different dependencies are handled correctly.
• A function that returns all components of a specified type_name in an sfg.
• Test cases covering both of above.
• Removal of the old precedence_graph and schema files.
• Updated class diagram with new precedence_graph functions.
• The inputs_required_for_output() implementation for SFG is now iterative and is nolonger is recursive.

Closes #25 (closed)