Cysgp4

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Add a benchmark section to the User Manual.

Bugfixes
~~~~~~~~~~
- The pytest benchmarks were not working properly (so we had them skipped),
because it was not clear how to check if the pytest-benchmark plugin was
installed in the first place. This is now clear: can add a config routine
to `conftest.py`.

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Finish the User Manual.

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New Features
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- Add two example tutorial notebooks.
- Introduce several helper/utitlity functions, e.g. to create your own TLEs
from orbital parameters.

Bugfixes
~~~~~~~~~~
- Include tests in (installed) package, such that test runner can find them.
- Fix github pages rendering, where css/js wasn't found in static directories
with a leading underscore (an empty .nojekyll is necessary!)

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Bugfixes
~~~~~~~~~~
- Really show README on PyPI.

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Bugfixes
~~~~~~~~~~
- Show README on PyPI.
- Twine upload didn't work for Python3.5.

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Add wheels for MacOS and Windows.