Merge branch 'issue-474' into develop

src/main/java/org/orekit/files/ccsds/definitions/Units.java

...files/ccsds/ndm/odm/ocm/CovarianceHistoryMetadataKey.java

...main/java/org/orekit/files/ccsds/ndm/odm/ocm/OcmFile.java

...in/java/org/orekit/files/ccsds/ndm/odm/ocm/OcmParser.java

...orekit/files/ccsds/ndm/odm/ocm/OcmSatelliteEphemeris.java

...rg/orekit/files/ccsds/ndm/odm/ocm/OrbitDetermination.java

...orekit/files/ccsds/ndm/odm/ocm/OrbitDeterminationKey.java

...kit/files/ccsds/ndm/odm/ocm/OrbitDeterminationWriter.java

...files/ccsds/ndm/odm/ocm/OrbitStateHistoryMetadataKey.java

...rg/orekit/files/ccsds/ndm/odm/ocm/PhysicalProperties.java

...orekit/files/ccsds/ndm/odm/ocm/PhysicalPropertiesKey.java

...kit/files/ccsds/ndm/odm/ocm/PhysicalPropertiesWriter.java

.../orekit/files/ccsds/utils/lexical/KvnLexicalAnalyzer.java

src/main/java/org/orekit/utils/units/PrefixedUnit.java

src/main/java/org/orekit/utils/units/Unit.java

...ava/org/orekit/files/ccsds/ndm/odm/ocm/OcmParserTest.java

src/test/java/org/orekit/utils/units/UnitTest.java

src/test/resources/ccsds/odm/ocm/OCMExample2.xml

src/test/resources/ccsds/odm/ocm/OCMExample4.txt