Add output method for base driver class.

[maddenp-noaa]

AC

• Add default instance @property method output() to Driver base class.
• Define uwtools.exceptions.UWNotImplemented and raise it from the default output().
• Test default output() via ConcreteDriver* test subclass(es) in test_driver.py.
• Add unit test to each existing driver test module (test_chgres_cube.py et al.) where the tested driver subclasses Driver. The test should call output() and assert that the expected exception is raised.
• Update all driver unit tests to include output in the list of methods inherited from the base class.

Description

This method will serve as the default behavior for when a subclass has not yet implemented its own output(). It will report to the user that the method hasn't yet been implemented and will raise a UWNotImplemented exception.

Consider implementing, in addition to the API-callable output() method, something like

@external
def  show_output(self):
    yield self.taskname("output")
    print(json.dumps(self.output, indent=2, sort_keys=True))
    yield asset(None, lambda: True)

that can be called from the CLI to show the output() results on stdout. This could then be called as part of the CLI documentation generation to show example output in the CLI docs. (The doc would should be done in per-driver tickets, not here.) The output might look something like

>>> import json
>>> show = lambda x: print(json.dumps(x, indent=2, sort_keys=True))
>>> show("/path/to/foo")
"/path/to/foo"
>>> show(["/path/to/foo1", "/path/to/foo2"])
[
  "/path/to/foo1",
  "/path/to/foo2"
]
>>> show({"foo": ["/path/to/foo1", "/path/to/foo2"], "bar": ["/path/to/bar1", "/path/to/bar2"]})
{
  "bar": [
    "/path/to/bar1",
    "/path/to/bar2"
  ],
  "foo": [
    "/path/to/foo1",
    "/path/to/foo2"
  ]
}

This way, users could use the CLI to see what the output of their driver invocation will look like, and examples can be programmatically included in our CLI docs.