Allow series outputs for single-column `NestedSeries.explode`, `NestSeriesAccessor.to_flat`, `NestSeriesAccessor.to_lists` results

[dougbrn]

Very related to #381

These functions will always return dataframes, but in cases where there's only a single sub-column I think it would be nice to have these return a series instead. Motivated by the deprecation of get_flat_series, there's a performance consequence (see ~50% slowdown in some cases) to using these functions as a replacement that is due to the unnecessary dataframe casting involved in to_flat and/or explode.

[dougbrn]

Just adding a rough snippet that works for to_flat, by adding this there's performance parity between get_flat_series(col) and to_flat(columns=[col])

if len(flat_series) == 1:
    return flat_series[list(flat_series.keys())[0]]