Add single Series to MultiSeries object

[kcpevey]

We need a way to add a single Series to a MultiSeries object.

Perhaps something like this:
MultiSeries.add(Series)
or
MultiSeries.add([Series1, Series2])

[CommonClimate]

That's an incremental improvement over our current process, which we can easily do ourselves. We'd be more interested in an overloaded method, so that ms + ts1 + ts2 would append the Series objects ts1 and ts2 to an existing MultipleSeries object ms

[CommonClimate]

I've tested this and it works well