WIP on #517.

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

.../orekit/models/earth/GlobalPressureTemperature2Model.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

src/test/java/org/orekit/Utils.java

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

...kit/models/earth/GlobalPressureTemperature2ModelTest.java

src/test/resources/gpt2-grid/gpt2_15.grd

src/test/resources/gpt2-grid/gpt2_5.grd

src/test/resources/gpt2-grid/gpt2_5_extract.grd