add Systematics::sys2ntbsys()
bugfix: compute trig and btag sf for all obj-syst variations

This bug was due to a remnant of some premature optimization.
The idea was that computing the b-tag and the trigger scale
factors can be time-consuming, so they should be recomputed only
when doing the corresponding variation. However, because the
weight components are reset to 1.0 at each systematic iteration,
the btag and trig sf were 1.0 for all object variations that
did not involve trig and btag, giving yields that were always
larger than the nominal one.

The time saved with the optimization is negligible
(running on 177501 with two nom + 2 syst)
```
        with buggy opt.   w/out optimization
real    0m17.790s                 0m18.251s
user    0m17.199s                 0m17.747s
sys     0m0.316s                  0m0.372s
```

The total yield is now sensible

```
$ grep  " nominal weight " trashbin/debug_syst.log | head -n 1
run 195847 event 10047 nominal weight 6.9069515927e-05, components  susynt: 0.0961285 lepSf: 0.991777 btag: 0.985395 trigger: 0.61865 qflip: 0.00118841 fake: 1

for X in nominal EES_Z_UP EES_Z_DN;
do
    echo $X : `grep  " ${X} weight " trashbin/debug_syst.log | awk '{ total += $7 } END { print total }'`
done
nominal : 48.5742
EES_Z_UP : 49.0648
EES_Z_DN : 48.4631
```