S1etad

--------------------------

* 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

--------------------------

* Improve compatibility with GDAL 3.x.

--------------------------

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

--------------------------

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

--------------------------

* 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

--------------------------

* Implement ETAD-DLR-PS-0014 "Product Format Specification" v1.2.
* The burst catalogue initialization and the
:meth:`s1etad.product.Sentinel1Etad.s1_product_list` method have been
re-implemented to exploit NetCDF instead of XML (30% faster product loading).
* All notebooks have been update to use the new demo products
(with updated format).
* The back-geocoding implementation has been simplified and improved
(caching of ECEF coordinates during guess computation).
* Fixed :meth:`s1etad.product.Sentinel1Etad.iter_bursts` in case of empty
``selection``.
* Fixed KMZ generation in case of missing slices.
* Added min/max range time attributes to :class:`s1etad.product.Sentinel1Etad`
class.
* New notebook providing a basic step by step guide to perform the
correction of a single Sentinel-1 SLC burst with the timings provided
by the S1-ETAD products.
* Now the :meth:`s1etad.product.Sentinel1Etad.s1_product_list` always returns
a list. Previously a string was returned in case of single swath.
* Fixed the :meth:`s1etad.product.Sentinel1Etad.query` method in the case
in which the ``product_name`` parameter is used to search for S1 Standard
("S") products.
* Always use the `Sphinx RTD Theme <https://sphinx-rtd-theme.readthedocs.io/>`_
(also for local builds).