Added a way to recover supported frequencies in GNSS antennas.

src/main/java/org/orekit/errors/OrekitMessages.java

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

...ces/assets/org/orekit/localization/OrekitMessages_da.utf8

...ces/assets/org/orekit/localization/OrekitMessages_de.utf8

...ces/assets/org/orekit/localization/OrekitMessages_el.utf8

...ces/assets/org/orekit/localization/OrekitMessages_en.utf8

...ces/assets/org/orekit/localization/OrekitMessages_es.utf8

...ces/assets/org/orekit/localization/OrekitMessages_fr.utf8

...ces/assets/org/orekit/localization/OrekitMessages_gl.utf8

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...ces/assets/org/orekit/localization/OrekitMessages_no.utf8

...ces/assets/org/orekit/localization/OrekitMessages_ro.utf8

src/test/java/org/orekit/errors/OrekitMessagesTest.java

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