Read: Close Oldest Iterations

In reads of very large data series, it is common to run into ulimits.

One convinient way to support scripting that we could offer is to implement a user-facing option on how many iterations to not call `.close()` on when opening a new iteration in non-streaming read mode.

That way, we could clean up the oldest iterations we opened when opening a new one while looping through series for analysis and the user does not need to keep track. Such an option would also make it simpler for openPMD-viewer et al. to implement iterations through series.