NamedTuple return values for `plottable_array_and_units` and others

[ken-lauer]

plottable_array_and_units returns 5 things:

    Returns
    -------
    scaled_array : np.ndarray
    factor : float
    units_str : str or None
        Display units of the scaled values.
    xmin : float
    xmax : float

This, and any other function that returns more than 2ish items, should be converted to a namedtuple.
That would mean you could access scaled_array as plottable_array_and_units()[0] as you would currently or plottable_array_and_units().scaled_array.

This is generally good practice as confusion about positional order of return values is similar to having too many positional arguments in a function. It's better to name them.