Added subset (filter) test, sparse array addition test
drop=FALSE now default to hew closer to standard R behavior; added recycle-like ability to multiply v * X, where v is a vector and X is a matrix such that length(v) = nrow(X)--this has the effect of row scaling the matrix by v. Note that general vector recycling is not supported.
added lag macro, fixed a minor bug in build
Fixed a big in internal build_attr_schema function
Working on partial outer join support