Set total data loaded for the PSD calculation from frame file to be `psd_duration`
There was a typo in the way the strain data was set for the PSD calculation when reading from a frame file. In line 654 of detector.py, t2 was set such that the total duration of the loaded data was psd_offset instead of psd_duration. I've just changed the definition of t2 so that the total data loaded is psd_duration.
