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.