Finalized IGS ANTEX antenna models file handling.

...ain/java/org/orekit/gnss/GNSSAttitudeProviderFactory.java

src/main/java/org/orekit/gnss/antenna/Antenna.java

src/main/java/org/orekit/gnss/antenna/AntexLoader.java

src/main/java/org/orekit/gnss/antenna/FrequencyPattern.java

src/main/java/org/orekit/gnss/antenna/OneDVariation.java

...in/java/org/orekit/gnss/antenna/PhaseCenterVariation.java

src/main/java/org/orekit/gnss/antenna/ReceiverAntenna.java

src/main/java/org/orekit/gnss/antenna/SatelliteAntenna.java

...in/java/org/orekit/gnss/antenna/SatelliteAntennaCode.java

src/main/java/org/orekit/gnss/antenna/TwoDVariation.java

src/main/java/org/orekit/gnss/antenna/package-info.java

src/main/java/org/orekit/overview.html

src/site/markdown/index.md

src/site/xdoc/changes.xml

src/test/java/org/orekit/gnss/antenna/AntexLoaderTest.java