vectorize outer loop

ok, now vectorizing outer loop.

Converting 1000 events now takes 0.8 s (!)

time python convertNanoToHDF5_L1triggerToDeepMET.py -i data/perfNano_TTbar_PU200.110X_set0.root -o data/perfNano_TTbar_PU200.110X_set1.h5   -N 1000

real	0m0.832s
user	0m0.708s
sys	0m0.117s