add hook to make orekit_jpype pyinstallable

This adds a Pyinstaller hook to the orekit_jpype library, so that users can more easily package their Python app depending on Orekit as an executable. This is necessary because the jar files are by default not packaged.

Documentation was added to README.md.

A test case is available (which builds the executable and runs a minimal test) and can be run with python -m PyInstaller.utils.run_tests --include_only orekit_jpype._pyinstaller -v. TODO: when the CI pipeline is operational, add this test to the pipeline.

A similar merge request is proposed for the orekit data repository.

Heavily inspired from a similar hook for the jpype jars in https://github.com/jpype-project/jpype/pull/877