[UI] Switchable Widget.

[YooSunYoung]

Related to #28

We want a widget that wraps another widget.
If the switch is on, it should set the value from the child widget to the workflow,
if it's off, it should not be set at all.

We can either use checkbox or a accordion : D

[YooSunYoung]

import ipywidgets as widgets
from ess.reduce.parameter import Parameter


def make_text_widget(param: Parameter) -> widgets.Text:
    return widgets.Text(
        value=param.default,
        description=param.name,
    )


class OptionalWidget:
    def __init__(self, widget: widgets.Widget):
        self.underlying_widget = widget
        self.optional = widgets.Checkbox(description="Optional")
        self.widget = widgets.HBox(
            [
                self.underlying_widget,
                self.optional,
            ]
        )

    def value(self):
        if self.optional.value:
            return None
        return self.underlying_widget.value


sample_param = Parameter("sample", "Sample", "sample")


def wrap_as_optional(widget: widgets.Widget) -> widgets.VBox:
    return OptionalWidget(widget).widget


wrap_as_optional(make_text_widget(sample_param))

This is one of the ideas of optional widget.

I'll open a PR of more trimmed version once param-class is polished first so that it doesn't become too big...?