Use directly navigation links to store TimeSpanMap entries.

src/changes/changes.xml

src/main/java/org/orekit/attitudes/AttitudesSequence.java

src/main/java/org/orekit/files/sinex/SinexLoader.java

src/main/java/org/orekit/gnss/clock/RinexClock.java

src/main/java/org/orekit/utils/TimeSpanMap.java

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

src/test/java/org/orekit/utils/TimeSpanMapTest.java