JYMAG 3.1 released!

This is mostly a maintenance release. No new features are present.
Fixes in the user interface.
Defect fixes.
Improvements in error handling.
Many fixes in coding style and quality.
Improvements in the testing area and many new tests added (current line coverage: 67% and 33% in headless mode).
Updates in the build system.
Updates in the documentation.
Verified to start under Java version 21.