BSV table functionality

[hafen]

Add BSV table functionality. When a ddo/ddf object has between-subset-variables attached to it, make this an attribute that gets updated with updateAttributes, such that all the BSVs are pulled together into a bsv data frame, with a column including the key (or digested key for more complex keys). We can then use this table to interact with the data (pull out subsets that have interesting BSV values).