FrqeuencySeries read from LIGO-LW XML document has incorrect f0

Created by: lpsinger

When a FrequencySeries is read from a LIGO-LW document, it has an incorrect f0.

>>> import lal.series
>>> import glue.ligolw.utils
>>> from gwpy.frequencyseries import FrequencySeries
>>> psds = lal.series.read_psd_xmldoc(glue.ligolw.utils.load_filename('O3_HLV.xm
l', contenthandler=lal.series.PSDContentHandler))
>>> psd = psds['H1']
>>> psd.f0
10.0
>>> psd = FrequencySeries.from_lal(psd)
>>> psd.f0
<Quantity 10. Hz>
>>> psd.frequencies[0]
<Quantity 10. Hz>
>>> psd = FrequencySeries.read('O3_HLV.xml', instrument='H1')
>>> psd.f0
<Quantity 0. Hz>
>>> psd.frequencies[0]
<Quantity 0. Hz>