Cool curves and stuff

src/main/java/no/nibio/vips/entity/WeatherObservation.java

src/main/java/no/nibio/vips/model/maths/NonLinearCurves.java

src/main/java/no/nibio/vips/util/SolarRadiationUtil.java

src/main/java/no/nibio/vips/util/WeatherElements.java

...t/java/no/nibio/vips/model/maths/NonLinearCurvesTest.java

src/test/java/no/nibio/vips/util/SolarRadiationUtilTest.java