Converter CLI

We should write a small command line interface (CLI) tool that provide converters from one backend to another.

E.g. if all backends are enabled, convert openPMD *series* of files:
```bash
openPMD-convert -o h5 path/to/simOutput_%T.bp
```

A cool extra option would be to add a flag that *mocks* the heavy data entries, e.g. for constant records with zero-value. This might be very useful for validation and meta-data scraping.

We should also add an optional parameter `-i|--iteration ` to only convert a single iteration instead of the full series.

We should also add an optional parameter to change to group/file based iteration format during the convert.

We should also add an optional parameter with a different path for the output.