Base fragments for pdf/systematics in most MG tests/examples
As discussed in [AGENE-1614], the MG test JOs now use base fragments.
Exceptions are the three simplest tests with dsid 950100-950102 (I will update the MG twiki and make clear that these aren't examples one should follow, just tests) and the more involved SUSY JOs.
Also fixed issue with reweight test JOs.