read_sixte_lc: Set time default from end to center of bin

* The time column was set to dt*[1:n], which represents the end of the
  time bin
* Change it to the center, i.e. (dt*[0:n-1]+dt*[1:n])/2=dt*[1:n]-dt/2
* Also provide a time_lo, time_hi in the return struct in case the
  lightcurve shall be plottet with ISIS' hplot routine