Should methods that return series return timespectra just like slicing?

[hughesadam87]

Noticed that methods like np.cov() will return a series of values given a timespectra (timeseries actually). Seems like the place to fix this would be in _dfgetattr in TimeSpectra.

Tried putting this in before hte if isintance(out, DataFrame) line:

    ### If operation returns a dataframe, return new TimeSpectra
    if isinstance(out, Series) or isinstance(out, TimeSeries):
        out=DataFrame(out)        

Caused some errors that I didn't look into, but I think it might not work just cuz some instances of series are necessary. Like storing the references and dark spectra and such.