TimeSeriesDict should include class method to instantiate from NDS2 buffer list

Created by: jrollins

I guess it should look something like this:

@classmethod
def from_nds2_buffers(cls, bufs):
    tsd = cls()
    for buf in nds2_bufs:
        tsd[buf.channel.name] = TimeSeries.from_nds2_buffer(buf)
    return tsd

Instantiation of this class is not well documented, though, so I'm not sure if there's a better way. Presumably the appropriate code exists internally for handling the fetch() class method.