Minor improvements for release process

This MR contains several small quality-of-life improvements to the release process. I think after that we could release orekit_jpype to PyPi?

• Download JARs using maven. The Orekit&Hipparchus&Rugged versions can be specified in pom.xml
• the render_stubs.py script was extended (and renamed) to delete the old JARs (to avoid mixing multiple versions of Java artifacts), download the new JARs, delete the old stubs, re-generate the stubs
• the Javadoc JARs are also downloaded using maven, but now gitignored, because they are only needed to generate stubs and not for using orekit_jpype. This reduces the wheel size by more than half, now it's only 7.1 MB.
• update README.md, especially the dev instructions how to prepare, build and upload a new release
• Bump package version to 12.0.2.0
• Add unit tests to increase test coverage of orekit_converters.py and pyhelpers.py and orekit_jpype.py
  • The tests that start the JVM are split into several files meant to be run separately because the JVM cannot be restarted (known jpype/JNI limitation)
• To make the tests pass, I had to do some changes in the three Python files mentioned above, for instance:
  • pyhelpers: big overhaul of JArray (1D&2D) and elevation mask conversions
• add jvmpath arg to initVM (passed directly to jpype) to use custom JVM. This allows to use jdk4py without having to use a JAVA_HOME environment variable
• many small syntax fixes to please the flake8 linter
• Big changes to the CI pipeline:
  • the CI pipeline now uses official Python debian docker images, with jdk4py via pip to provide a JDK
  • A matrix of several Python versions (3.9 to 3.12) is used for testing
  • a new step was introduced in the CI pipeline to try building the wheel
  • the CI pipeline will keep the built sdist and wheel as artifacts for one day, just in case. And also the test coverage and code quality reports for one month.