Fix a bug that was introduced in GWCelery 0.12.1 that broke BAYESTAR
localizations for PyCBC events. In 0.12.1, the BAYESTAR data handling
was changed to merge together the contents of the coinc.xml and
psd.xml.gz files into a single XML document so that BAYESTAR was not
sensitive to the order in which the two files were passed to it. PyCBC
includes the PSD data in its initial upload, and so its psd.xml.gz file
is just a copy of coinc.xml. Merging the two documents together
resulted in a single file with two copies of every LIGO-LW table, which
broke subsequent parsing.

Fix this by adding a special case for PyCBC to download the coinc.xml
only. This has the nice side effect of reducing the latency for PyCBC
events because it is no longer necessary to wait for the additional
GraceDB REST API calls involved in uploading and download the
additional file.

Fixes #341.