Add atomic-add, incr() calls add(1)
Add accessor

better comments
Add the ignored airline cols to the test file
Overhaul the memory access of GBM hot code

All rows, all cols, accumulate histograms.  This is the hot hot inner
loop of GBM, so we do some non-standard optimizations.  The rows in this
chunk are spread out amongst a modest set of NodeIDs/splits.  Normally
we would visit the rows in row-order, but this visits the NIDs in random
order.  The hot-part of this code updates the histograms racily (via
atomic updates) - once-per-row.  This optimized version updates the
histograms once-per-NID, but requires pre-sorting the rows by NID.
Speed on bigger data is 10x or 20x faster.
nnids[] array holds node-relative indices (_leaf pre-subtracted).
Merge branch 'master' of github.com:0xdata/h2o