Series Open Performance

We should investigate and profile our performance of `Series` open (especially with `READ_ONLY` and `READ_WRITE`, but also with `CREATE` on truncation: #483).

Most crucial with file-based series, I have the subjective feeling they are slower than they need to be, already in serial mode.

We might need to clearly define also in this step, what we check and assume when opening a series (aka [contract](https://en.wikipedia.org/wiki/Design_by_contract)). The less assumptions the better, if you ask me. For example, I would not expect the data of each iteration nor it's names or layouts to be required to match. (Although some applications, such as our [openPMD-viewer](https://github.com/openPMD/openPMD-viewer/) GUI, will benefit if we later on provide an auxiliary `scan`-like list summary for records in all iterations.)

### Resources

Maybe some useful resources:

- [`scandir()`](https://geoff.greer.fm/2012/09/03/profiling-ag-writing-my-own-scandir/) in [`ag`](https://github.com/ggreer/the_silver_searcher)
- [benchmarking idea](https://geoff.greer.fm/2012/08/25/the-silver-searcher-benchmarking-revisions/)
- [`tracefile`](https://unix.stackexchange.com/questions/18844/list-the-files-accessed-by-a-program) (`strace` maybe expand to measure `time` spend; or even easier: add time log #462 to profile our `auxiliary::Filesystem` operations)