Add 'mask' function for Series

[adamklein]

To return same series with values masked out (with missing values). Maybe

let mask (keys: 'K seq) (series:Series<'K,'T>) =

Or maybe

let mask (maskFn: 'K * 'T option -> bool) (series: Series<'K,'T>) =
    let maskFn' (kvp: KeyValuePair<'K, OptionalValue<'T>>) = 
        if not (maskFn (kvp.Key, OptionalValue.asOption kvp.Value)) 
        then kvp.Value else OptionalValue.Missing       
    series.SelectOptional(Func<_,_>(maskFn'))

@dshiber