add method replaceNULLS in WeatherUtil

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

.../java/no/nibio/vips/ipmdecisions/DataTransformerTest.java

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

src/test/resources/replaceNULLtestDestination.json

src/test/resources/replaceNULLtestSource.json