HovmoellerDomain and Feature classes followed design doc. Add extractHovmoellerFeature in AbstractGridDataset. Basic unit test in RectiLinearGridDatasetTest.
Add plotHovmoellerFeature in Charting.java and extractHovmoellerFeature in AbstractGridDataset.java. Add testExtractHovmollerFeature in RectiLinearGridDatasetTest.java. New HovmoellerPlotTest.java.