Annotate method inputs and outputs using typing

[cmungall]

For example:

def parse(input):
    """
    Parse a set of KGCL command separated by next-line operator.

    Returns instantiated dataclass objects from model.kgcl.
    """
    statements = input.splitlines()
    parsed = []

    for s in statements:
        parsed.append(parse_statement(s))
    return parsed

The arguments and the output should be typed

[cmungall]

Mostly done in #50