fixed numerical issue in eccentricity computation

src/main/java/org/orekit/orbits/KeplerianOrbit.java

src/site/xdoc/changes.xml

src/test/java/org/orekit/orbits/CartesianParametersTest.java

src/test/java/org/orekit/orbits/CircularParametersTest.java

...est/java/org/orekit/orbits/EquinoctialParametersTest.java

src/test/java/org/orekit/orbits/KeplerianParametersTest.java