Added loading of RINEX observation files (versions 2 and 3).

pom.xml

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

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

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

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

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

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

src/main/java/org/orekit/time/BDSScale.java

src/main/java/org/orekit/time/IRNSSScale.java

src/main/java/org/orekit/time/TimeScalesFactory.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

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

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

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