Fix possible bug in modify_param_card
I ran into a problem while adding only new masses to madgraph param cards. At
doneParams is initialized as an empty dict, but later when building leftovers, it assumes that "MASS" is already a key of doneParams.
These changes should mean that if you're only adding new particle masses, you don't get a KeyError.
Originally reported in https://its.cern.ch/jira/browse/AGENE-2042