Ascat

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- Fix datatype cast issue

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- Fix typo in flagfield name

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- Add keyword to ignore ASCAT Level 1b flag noise out of limits

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- Update file handling read_period
- Refactor xarray readers/writers into SwathFileCollection, CellFileCollection,
and CellFileCollectionStack
- Add tutorial for xarray readers/writers
- Add CLI for aggregating ASCAT swath data
- Add CLI for regridding ASCAT swath data to a regular lat/lon grid

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- Add str to path conversion for H SAF FTP download

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- Update interface of file handling module