Fix timstep issue with Nek5000 file series.

When opening a Nek5000 fileseries, only the 1st timestep had correct time
values, all the others were 0. This patch fixes that issue based on the code in
VisItBridge 2.6.3. Basically, we needed to iterate over the files during
initialization to determine the vailable timesteps.
Merge branch 'nex5000-timesteps'