Fixed various issues in geomagnetic fields models:
- GeoMagneticField.getDecimalYear() returned a slightly wrong result: e.g. for 1/1/2005 returned 2005.0020 instead of 2005.0,
- GeoMagneticFieldFactory.getModel() returned wrong interpolation near models validity endpoints,
- GeoMagneticField.transformModel(double) method did not check year validity.
Added more unit tests and adapted existing tests for IGRF/WMM with
sample values / results as they have changed slightly.
Fixes issue #178.