Subset data sets

[TonyNolan]

For example on the AusWeather dataset, picking a different city to make a smaller dataset from the bigger one

[gjwgit]

Add a SUBSET feature on the TRANSFORM tab, probably before the CLEANUP feature. The layout will follow the standard of other pages with a button to GENERATE SUBSET. The interface will allow variable/relation/value triples to be specified combined with AND/OR. GENERATE will run appropriate R code to construct the subset so specified. The remainder of the page has a description on the left and a relevant image on the right.