Method for querying if a selected timestep is valid, or which timesteps exist

[Twinklebear]

For viewer or analysis applications it would be useful to be able to get a list of which timesteps exist in the file, or to check if the selected timestep is valid before trying to read the data. Currently if you pick a non-existing timestep and try to load it you get a generic PIDX_err_close which isn't informative