Changed computePSD() to use fixed data lengths (86400 sec) --> this
*should* get rid of the null pointer exceptions James keeps finding.
Also modified several metrics to handle BHN,BHE when BH1,BH2 are not
found.
Hopefully fixed null pointer exceptions occurring in cases where there are
data files on disk but no datasets have been loaded, and also in
MetricData.getPaddedDayData when the dataSet end time slightly exceeds 86400 secs.