Added a way to manage clock corrections from GPSPropagator.

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

...propagation/analytical/gnss/ClockCorrectionsProvider.java

...rekit/propagation/analytical/gnss/GPSOrbitalElements.java

src/site/xdoc/changes.xml

...t/propagation/analytical/gnss/GPSOrbitalElementsTest.java

...orekit/propagation/analytical/gnss/GPSPropagatorTest.java