Add blockwise_inner_join to deal with more complex data structure

For example, in the most popular BaBar data dump the structure(BtaTupleMaker). The output structure is something along this line.

In case of B->Dpi->K pi pi, where there are multiple candidate for B

1Event ==> [B block fields] [D block fields] [K block fields]
to get DMass that's the daughter of 2nd B user can do
DMass[Bd1Idx[1]]

to get KMomentum that's daughter of D that's a daughter of 2nd B user can do
KMomentum[Dd1Idx[Bd1Idx[1]]]

This actually emulate event-wise inner join. This function just flatten the array using inner join.
See example at http://nbviewer.ipython.org/urls/raw.github.com/piti118/babar_python_tutorial/master/notebooks/Extra_02_Navigating_BtaTupleMaker.ipynb