series only works on seas_multi if they are a composite

[HomoCodens]

See https://github.com/christophsax/seasonal/blob/main/R/series.R#L330

I would (naively) have assumed it to return a list of the respective series, similar to how the component series are also represented in the series field of the model object.

I assume the rationale was that series should only return different series components for a single time series? Outright throwing an error surprised me a little.