Provide methods to easily convert old stored pulses

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so that they can be loaded using the new serialization routines

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Branch issues/250_serialization_conversion_script introduces methods convert_stored_pulse_in_storage and convert_pulses_in_storage to automatically convert pulses stored using the old serialization routines from a source storage backend to a destination storage backend using the new serialization routines.

Should maybe have some more tests to ensure that converted pulses are really identical (although current tests should in principle already guarantee that? check)