Added support for RINEX 3.04 files.

src/changes/changes.xml

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

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

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

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

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

src/test/resources/rinex/brca083.06o