Allow pandas Series in ps.forecast and transform to 1D DataFrames internally

[raoulcollenteur]

Describe the proposed feature

Right now a dataframe is expected, but for user-friendliness it would be good to allow for Series as well and internally convert these to 1D DataFrames.

Expected usage

Now this is required

fc = {
    "rch": [
        pd.DataFrame(scenarios),
        pd.DataFrame(scenarios),
    ]
}

where scenarios is a pandas.Series.

Additional context

Add any other context about the feature here.

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