Example reweights

Hi Burt and Ted,

 I really like the idea that Burt introduced with reweights, and I was trying to write an example as Ted suggested a few days ago. However, I must be missing something...
Here is what I have done:
- defined a secondary calculable with the ratio data/mc of N_trk for the first vertex
- defined a simple calculable that contains three weights https://github.com/davidegerbaudo/supy-minimal-example/blob/example-reweights/calculable_example.py#L8
- here https://github.com/davidegerbaudo/supy-minimal-example/commit/28b17e0612328b77c921c99beeef102106627f57#L0R17 I try to use reweights to build the nTrk distribution with the tree weights

However, when I run I get a warning
` !! No cache: /tmp/gerbaudo/example_analysis/cache_nTracksV0Ratio.root`
I still get the three histograms, but only one (the one with constant weight 2.0) is different.
I must be doing something silly.

I also have another question about the usage of `reweights`: I was thinking that it would be very useful, for example, when one wants to compare the distributions for some nominal case with the +/-1sigma syst variations. Was that the purpose? if so, shouldn't we add a parameter specifying for what samples the variation should be applied (e.g. one might want to keep the data with the nominal weight, while now the weights are applied to all samples...)

Thanks,
Davide