megaplot ` get_waveform_bigfile` and ` get_waveform` function calls need updating

A few lines of the megaplot script needs updating after the recent changes to accommodate the new file naming convention. Specifically, older versions used to have get_waveform(filename,ifo) type calls. This has now changed to get_waveform(filename) type calls as the filename variable subsumes the ifo variable ever since Tyson's updates.