Python: Improve integration with smart_open

[pspoerri]

The GEDS smart_open integration has been extended so that additional GEDS configuration options are supported:

• GEDS_AVAILABLE_STORAGE: Configure the storage threshold for GEDS.
• GEDS_AVAILABLE_MEMORY: Configure the memory threshold for GEDS.
• GEDS_RELOCATE_WHEN_STOPPING: Relocate files in GEDS to a configured object store when GEDS ist stopped.

Changes to the smart open integration:

• GEDS is now stopped when the application exists.
• relocation is exposed as an API.
• open(file, mode='wb') now always creates a new file in GEDS
• open(file, mode='ab') does an explicit copy of the file if the file is not writable (e.g. on a remote node).
• The Python API has now been improved so that GEDS read calls directly allocate a writable Python buffer.

[pspoerri]

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