Time series fetch seems to have trouble with with odd prefixes

Created by: areeda

The following code snippet fails with the latest pull from dev, it did work in previous versions:

from gwpy.timeseries import TimeSeries
chan = 'LVE-EX:X3_810BTORR.mean,m-trend'
dur = 36000
start = 1072539420
data = TimeSeries.fetch(chan, start, start+dur, verbose=True)

produces the error:
```AttributeError: 'Channel' object has no attribute '_ifo' ```

This is one of the examples in gwpy-plot documentation.  The plot from a previous version looks like: 

https://ligodv.areeda.com/gwpy/cli_examples/cli-ts-02.png