make the `._has_subplots` method from `go.Figure` accessible

In some cases, when writings callbacks that only update a subplot of a Figure, it is useful to know if the figure will accept an append_trace to a specific location, or if add_trace should be used instead.

I use this workaround (using a try/except isn't great):

    if fig._has_subplots():
        fig.append_trace(trace, 1, 1)
    else:
        _ = fig.add_trace(trace)

If I only use fig.append_trace(trace, 1, 1), I get Exception: In order to reference traces by row and column, you must first use plotly.tools.make_subplots to create the figure with a subplot grid.

If I only use fig.add_trace(trace), the layout gets messed up.

Could it be possible to make this method public?

go.Figure の _has_subplots の実装と、append_trace、add_trace、make_subplots に関する公開 Figure API を調査します。既存の subplot の動作を維持しながら、呼び出し側が try/except による回避策に頼らず、行と列による trace 参照が有効かどうかを判定するための公開チェックを使用できるようになれば、作業は完了です。