S1etad

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* Fix `shapely` warnings (Closes: 2 and 5).
* No longer use the deprecated `scipy.interpolation.interp2d` function.
* Update the documentation to use recent sample products.
* Code linting.
* Start using `pre-commit`_ for development.
* Setup automatic dataset download with `pooch`_.
* Basic unit tests added to the project.
* Enable CI jobs for linting and unit testing.


.. _`pre-commit`: https://pre-commit.com
.. _`pooch`: https://www.fatiando.org/pooch

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* Project moved to GitHub
* Move the project metadata to pyproject.toml
* Drop support for Python 3.6 and 3.7
* Enable CI workflows for GHA
* Fix several spelling errors
* Formatting (using `black`) and linting
* Do not use mutable default parameters
* Always set stacklevel for warnings
* Add badges to the `README.rst` file
* Update doc configuration
* Update copyright dates
* Add missing dependency on `pymap3d`
* Update the documentation of `back_geocode`
* Improve example for Use Case 4

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* Improve compatibility with GDAL 3.x.

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* Fix imports from ``typing`` (fall-back typing-extensions when necessary).

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* Add missing dependency form ``typing-extensions`` for Python < 3.8.

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* Merger functions now return masked arrays.
* The :class:`s1etad.product.Sentinel1EtadBurst` class now has two new
properties:

- :data:`s1etad.product.Sentinel1EtadBurst.vg`, the average zero-Doppler
ground velocity
- :data:`s1etad.product.Sentinel1EtadBurst.reference_polarization`,
a string identifying the reference polarization

and a new method:

- :meth:`s1etad.product.Sentinel1EtadBurst.get_polarimetric_channel_offset`
to get the constant time offset of a polarimetric channel w.r.t. the
reference one

* Support for ETAD-DLR-PS-0014 "ETAD Product Format Specification" Issue 1.5
(SETAP v1.6):

- new method (
:meth:`s1etad.product.Sentinel1EtadBurst.get_timing_calibration_constants`)
for retrieving timing calibration constants

* new :mod:`s1etad.ql` module for geo-coded quick-look images generation