Implement segment masking for TimeSeries objects

Created by: alurban

It would be reasonably straightforward to let users mask out unwanted segments (e.g., non-observing-mode data) from their TimeSeries, something like:

import numpy
from gwpy.segments import DataQualityFlag
from gwpy.timeseries import TimeSeries

# retrieve segment and timeseries data
flag = DataQualityFlag.query(...).active
data = TimeSeries.read(...)

# mask out non-observing time
indices = numpy.nonzero([x not in flag for x in data.times.value])
data.value[indices] = numpy.nan

(Of course the actual method should be more careful than to call data.times, but this is just an example.)