Elevation

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- Updated supported python versions.

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- Docs and dependencies updates.

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- Nothing changed yet.

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- Fixed project metadata.
- Update dependencies versions (using pip-tools).

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- Fix clean command.
Closes issue `21 <https://github.com/bopen/elevation/issues/21>`_.
- Add docstrings for all Pyhton API functions.
Closes issue `15 <https://github.com/bopen/elevation/issues/15>`_.

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- Revert the default product back to ``SRTM1`` by downloading from the
`Amazon Terrain Tiles on AWS servcie <https://aws.amazon.com/public-data-sets/terrain>`_.
Closes issue `18 <https://github.com/bopen/elevation/issues/18>`_.