Add support for multiple DDF stems files

The master configuration now specifies a list of DDF stems files instead of a single one. The current stems file is already quite long. This MR will allow to put different groups of processes into different files. The immediate goal is to allow extending the list of stems with 2017-specific entries (@mmahdavi this is one of the points we discussed).

I checked that runHZZanalysis runs successfully when given a DDF fragment and that launchAnalysis.sh HZZanalysis 1 creates configurations for batch jobs. This tests both C++ and Python interfaces to DDF stems.