Release 20150526

Summary of changes: 

CLOCK:

The latest version of the clock file contains all corrections during the mission up to 2015-05-26 and is required when using barycorr to
correct the arrival times of the individual photons. Performance continues to be mostly good (residuals < 2 msec, ~2 sigma) although there are single discontinuities in the timing offsets over the last month around day 1945 and 1949 (residuals are still small). 

The clock file is also available from the SOC public website:

http://www.srl.caltech.edu/NuSTAR_Public/NuSTAROperationSite/clockfile.php