Warn on unsupported openPMDextensions

```Series::openPMDextension() != 0``` may literally break the standard with everyhing we assume in this API w.r.t. metadata, structure and datasets.

Reading/writing any ```Series``` with ```Series::openPMDextension() != 0``` may succeed, depending on the used extensions (e.g. if they only _add_ information to the base standard). It is thus not advisable to stop execution when that condition is encountered.
Likewise, reading/writing may fail (e.g. if the extensiton _modifies_ or _removes_ information in the base standard). This will cause a failure at runtime.

We should thus at least notify the user (e.g. with a warning) that the current implementation does not support one or more of the requested/present extensions.