New non-standard unit

Created by: areeda

As we've learned, nonstandard units affect calculations on a timeseries and have to be handled separately. I found one that we do not handle that limits the ability to take spectrum. Here's an ipython snippet:

In [47]: chan = 'V1:DQ_VetoThr_VETO_DIFFP_TX10TY10'
In [48]: ts = gwpy.timeseries.TimeSeries.fetch(chan, 1213708756, 1213708756+10)

In [49]: ts.unit
Out[49]: UnrecognizedUnit(Counts.)

In [50]: a=ts.asd()
<snip>
ValueError: The unit 'Counts.' is unrecognized, so all arithmetic operations with it are invalid.

Note the period after "Counts."