Improved test coverage.

...n/java/org/orekit/files/ccsds/ndm/odm/CommonMetadata.java

.../java/org/orekit/files/ccsds/ndm/odm/oem/OemMetadata.java

...n/java/org/orekit/models/earth/atmosphere/NRLMSISE00.java

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

...g/orekit/files/ccsds/ndm/odm/oem/EphemerisWriterTest.java

...ava/org/orekit/files/ccsds/ndm/odm/opm/OpmParserTest.java

...va/org/orekit/models/earth/atmosphere/NRLMSISE00Test.java

...g/orekit/propagation/analytical/tle/TLEConstantsTest.java

src/test/java/org/orekit/time/AbsoluteDateTest.java

src/test/java/org/orekit/time/FieldAbsoluteDateTest.java

src/test/resources/ccsds/odm/opm/OPMExample7.txt

src/test/resources/ccsds/odm/opm/OPMExample8.txt