Added a way to retrieve Rinex header from the observations data set.

src/design/gnss-rinex-class-diagram.puml

src/main/java/org/orekit/gnss/ObservationDataSet.java

src/main/java/org/orekit/gnss/RinexHeader.java

src/main/java/org/orekit/gnss/RinexLoader.java

src/site/xdoc/changes.xml

...rekit/estimation/leastsquares/OrbitDeterminationTest.java

...t/estimation/sequential/KalmanOrbitDeterminationTest.java

src/test/java/org/orekit/gnss/RinexLoaderTest.java

...ls/java/fr/cs/examples/estimation/OrbitDetermination.java