Add standard GWOSC channel names to the defaults channels that are attempted

In the frame files distributed by GWOSC the channel names are DET:GWOSC-16KHZ_R1_STRAIN and DET:GWOSC-4KHZ_R1_STRAIN for the 16384Hz and 4096Hz sampled files, respectively. I've added these to the default list of channel names that are attempted if no channel name is supplied.