Pickle w/o Statics

[ax3l]

For the first implementation of multi-process (multi-node) Dask, we pickle objects like the Record and RecordComponents + their series.

The series is unpickled into a static function member, to avoid:

• double open/parsing for each object
• having to express lifetime of the series on the unpickled side with respect to the pickled object.

This works as a hack until you need to work with two series at a time.

https://github.com/openPMD/openPMD-api/blob/0.15.1/include/openPMD/binding/python/Pickle.hpp#L73-L77

[pordyna]

@ax3l I tried using dask delayed with openpmd-api for parallelization over iterations. I couldn't get it to work, I think, because dask wasn't able to pickle the series object. Would it make sense to add add_pickle to Series and Iteration?