A deferred load/store operation for a particle patch.
Our particle-patch API requires that users pass a concrete value for
storing, even if the actual write operation occurs much later at
series.flush().
So, unlike other record components, we cannot call .store_chunk() with
a buffer that has not yet been filled, but must wait until the point where
we actual have the data at hand already.
In short: calling .store() must be deferred, until the data has been fully
read from the sink.
This class stores the needed parameters to .store().