Added writing of OMM files.

src/main/java/org/orekit/files/ccsds/ndm/WriterBuilder.java

.../files/ccsds/ndm/odm/omm/MeanKeplerianElementsWriter.java

.../java/org/orekit/files/ccsds/ndm/odm/omm/OmmMetadata.java

...org/orekit/files/ccsds/ndm/odm/omm/OmmMetadataWriter.java

...java/org/orekit/files/ccsds/ndm/odm/omm/OmmTleWriter.java

...in/java/org/orekit/files/ccsds/ndm/odm/omm/OmmWriter.java

...va/org/orekit/files/ccsds/ndm/odm/opm/ManeuverWriter.java

...in/java/org/orekit/files/ccsds/ndm/odm/opm/OpmWriter.java

...ccsds/ndm/odm/opm/OsculationgKeplerianElementsWriter.java

...rekit/files/ccsds/utils/generation/AbstractGenerator.java

...va/org/orekit/files/ccsds/utils/generation/Generator.java

...ava/org/orekit/files/ccsds/ndm/adm/apm/APMParserTest.java

...ava/org/orekit/files/ccsds/ndm/odm/omm/OMMParserTest.java

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

src/test/resources/ccsds/odm/omm/OMMExample2.xml