Detect MEParam weights

[kdlong]

Extend parsing to detect MEParam weights. Most typical use case is a sample with MG reweighting, where the group is labed "mg_reweight," I believe.

Would also be good to capture explicit ME weights that are sometimes used in POWHEG. For the mW samples, these include "mass_variation," "width_variation," and "sthw2_variation"

[dteague]

Should be closed unless we want to do more with this...?