Consider workflow to generate from SM but reweight with custom UFO

[ajgilbert]

The issue with this is potentially that the SM generation will miss diagrams that are only present with the EFT, but worth testing since generating LO processes with higher jet multiplicities and a large number of SMEFT operators is taking a lot of integration time.

[ajgilbert]

Possible to do this by specifying NP=0 in the gridpack generation, then using change process [...] NP=1 in the reweight_card. Should document in the README.