Feature/memmap buffer dir

[belerico]

Summary

Describe the purpose of the pull request, including:

This PR fixes #61 by specifying the memmap directory, one for each rank, where all the buffers are saved when memmapped.

Type of Change

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• Bug fix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)

Checklist

Please confirm that the following tasks have been completed:

• I have tested my changes locally and they work as expected. (Please describe the tests you performed.)
• I have added unit tests for my changes, or updated existing tests if necessary.
• I have updated the documentation, if applicable.
• I have installed pre-commit and run locally for my code changes.

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