Extra options for make_gridpack.sh

[ajgilbert]

Add extra optional arguments to make_gridpack.sh:

• prefix - to add an extra string to the output gridpack name
• madgraph settings - commands to pass through to madgraph - allows modifying the param or run card on the fly

In addition, we will now set use_syst and systematics_program to false automatically, which we need to do for the weight transformation to work correctly (in future we might make this more flexible)

[ajgilbert]

After some discussion with @fstaeg, reverted the automatically seting of use_syst and systematics_program to false. The remaining changes seem uncontroversial, so merging now.