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* Fix loading of Sentine-1 orbit form string.
* New `strict` option (default: `False`) to enforce strict XML namespaces
checking in `xseof.load` and `xseof.from_string`.
* Improve docstrings to clarify that `lxml` is needed to use an `ElementTree`
as source for the `xseof.load` and `xseof.*.load` functions.
A dedicated unit test has been added also.
* Test coverage improved.