data_generation.py breaks if psd-dict is only partially filled

Bilby_pipe (specifically data_generation.py) breaks if psd-dict contains only some of the entries (e.g. you set up 3 detectors but pass only 2 psd files, wanting 3rd to be generated from data).

When run with partially filled prior-dict, the error occurs in line 321

318 @property
319    def psd_start_time(self):
320        """ The PSD start time relative to segment start time """
321        if self._psd_start_time is not None:
322            return self._psd_start_time
323        elif self.trigger_time is not None:
324            psd_start_time = -self.psd_duration
325            logger.info(
326                f"Using default PSD start time {psd_start_time} relative to start time"
327            )
328            return psd_start_time
329        else:
330            raise BilbyPipeError("PSD start time not set")

There is no attribute _psd_start_time

This is because when prior-dict is not None, then in line 153

150        if self.psd_dict is None:
151            self.psd_length = args.psd_length
152            self.psd_fractional_overlap = args.psd_fractional_overlap
153            self.psd_start_time = args.psd_start_time
154            self.psd_method = args.psd_method

self.psd_start time is never set, so the program breaks.