Pandas: data frame sub-selection

Finding the right block data out from a data frame was not elegant: one has to use different functions: like query, selection, loc, iloc, or directly use something like "df['x'] > df['y']", which is totally not chain-able.

Here is a few new features under Pandas 0.18.1 to simplify the flow of doing data frame selection.

Regardless of how difficult the selection logic is, because of the newly improved .loc, things start look neat!