Changed the ecliptic frame to a factory managed frame.

src/main/java/org/orekit/frames/EclipticProvider.java

src/main/java/org/orekit/frames/FramesFactory.java

src/main/java/org/orekit/frames/Predefined.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/EclipticProviderTest.java