openPMD write: close file

[ax3l]

Gives up the file handle and close the file after write.

Fix #187

Thank you for the report, @SchroederSa ! 👍

[ax3l]

Update: we found the issue was actually from the (HDF5) reader occupying the file handle, while interactively trying to write to it again.

Nonetheless, the series.close() is clean to add and avoids that we need to wait for Python's garbage collector to destroy series and then close it.