Merge branch 'master' into B1950-frame
Conflicts: src/main/java/org/orekit/frames/FramesFactory.java src/main/java/org/orekit/frames/MODProvider.java src/main/java/org/orekit/overview.html src/site/apt/architecture/frames.apt src/site/apt/index.apt src/test/java/org/orekit/frames/FramesFactoryTest.java