py systems: Bind OutputPort.Allocate(), DiagramBuilder.empty(), .GetMutableSystems()

[EricCousineau-TRI]

Motivation is to be able to make an "auto" printer system.

Using the FunctionSystem prototype I mentioned in Slack, I can do something like this:

def make_printer(cls, fmt=None, print=print):
    if fmt is None:
        fmt = "{}"

    def print_value(value: cls) -> None:
        print(fmt.format(value))

    printer = FunctionSystem(print_value, set_name=False)
    return printer


def connect_printer(builder, output, dt, fmt=None):
    cls = type(output.Allocate().get_value())
    printer = make_printer(cls, fmt=fmt)
    builder.AddSystem(printer)
    builder.Connect(output, printer.get_input_port(0))

...
# No redundant info necessary.
connect_printer(builder, system.get_output_port(0), 0.1)

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This change is 

[EricCousineau-TRI]

+@sherm1 for both reviews, please.
+(status: single reviewer ok)

Reviewable status: LGTM missing from assignee sherm1(platform) (waiting on @sherm1)

[EricCousineau-TRI]

Added .GetMutableSystems() because I would like to get an added system by name (rather than explicitly renaming it) during building time. This is the easiest route to do so in Python.

[sherm1]

platform and feature (sort of -- looks reasonable but my pybind expertise is insufficient to say whether the py::casts and keep_alive are the right incantations). I'm satisfied if you are though.

Reviewed 2 of 2 files at r1.
Reviewable status: commits need curation (https://drake.mit.edu/reviewable.html#curated-commits)

[EricCousineau-TRI]

Thanks! The per-element py_keep_alive is a bit conservative - I could have just done keep-alive on the returned list; however, if a user were to mutate that list for whatever reason, it may screw up the keep-alive-ness (e.g. the user deletes a system from the list, the keep alive no longer applies to that system, and could cause a read-after-delete segfault if the Python GC acts too early on the builder).