get_windows needs to have an argument to set TargetOnset

Right now the code is written in that function such that it's looking for the target onset to be present in the dataframe. For the studies that don't involve incorporating the message report, this isn't in there. The current work-around is to just set it as a value in the global environment, but that's fairly delicate and should be replaced with an argument inside the function that can be optionally set to a single value.