Merge branch 'issue-448' into develop

src/changes/changes.xml

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

src/main/java/org/orekit/forces/gravity/OceanTidesField.java

src/main/java/org/orekit/forces/gravity/SolidTidesField.java

...potential/CachedNormalizedSphericalHarmonicsProvider.java

.../forces/gravity/potential/ConstantSphericalHarmonics.java

.../org/orekit/forces/gravity/potential/EGMFormatReader.java

...n/java/org/orekit/forces/gravity/potential/Flattener.java

...org/orekit/forces/gravity/potential/GRGSFormatReader.java

.../orekit/forces/gravity/potential/GravityFieldFactory.java

...rg/orekit/forces/gravity/potential/ICGEMFormatReader.java

...kit/forces/gravity/potential/LazyLoadedGravityFields.java

.../java/org/orekit/forces/gravity/potential/Normalizer.java

...va/org/orekit/forces/gravity/potential/PiecewisePart.java

...forces/gravity/potential/PiecewiseSphericalHarmonics.java

...forces/gravity/potential/PotentialCoefficientsReader.java

...forces/gravity/potential/PulsatingSphericalHarmonics.java

...rces/gravity/potential/RawSphericalHarmonicsProvider.java

.../org/orekit/forces/gravity/potential/SHMFormatReader.java

...ces/gravity/potential/SecularTrendSphericalHarmonics.java