Get the weight for B relative production and decay modes and D decay modes

@tmrhombus @mcepeda

This codes contains a producer that reads from some text files the weights corresponding to the P6->P8 correction of the relative B meson production rates, decay ratesm and D decay rates. It puts in the MC ntuples a float that can be used in the analyzer. Note that this is a relatively simple approach that supposed that the D mesons come exclusively from the B meson decays (which is largely true after applying our usual selections). So these weights should be used *only* with Wbb, TTbar and perhaps single top, NOT with W*jets etc.... 

Note an additional thing: these weights are computed from P6 and P8 BB QCD samples only. I'll have to recompute them with TTbar samples to make sure that the dependency to the type of b-emission vertex (QED for TTBar) does not influence too much these weights. If this is the case, I'll need to put vertex-specific tables (not a big deal...)