rootio: implement fast BulkIO

[sbinet]

It would be great to implement fast BulkIO access for rootio.

this can be done in a few lines in python:

• https://github.com/scikit-hep/uproot

with quite the performances boost:

• https://github.com/scikit-hep/uproot#performance

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The first comparison is time spent opening the file and loading the TTree. This is relevant if you are executing a procedure on a large set of files (TChain). uproot is about 16 times faster.

	Time to open file
C++ ROOT	0.50 sec
uproot	0.03 sec

Next is the time to read nearly all branches, exactly the same as the test on page 11 of Jakob's talk. uproot is about 5 times faster.

	Time to read file	Event rate	Data rate
C++ ROOT	4.62 sec	1.9 MHz	230 MB/sec
uproot	0.93 sec	9.2 MHz	1160 MB/sec

Finally, we load only one branch (replacing TTree::GetEntry with TBranch::GetEntry in C++ ROOT to avoid a performance penalty). uproot is about 4 times faster.

	Time to read 1 branch	Event rate	Data rate
C++ ROOT	0.256 sec	33 MHz	260 MB/sec
uproot	0.064 sec	133 MHz	1020 MB/sec

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[vkuznet]

+1