Add DeltaMs-specific B2DXFitter changes

This MR implements

• a few new command line arguments for the script:
  • runMDFitter
    • allow specification of number of cpus used for the fit and the RooFit minimizer
  • plotMDFitter
    • allow to specify a plot prefix
  • prepareWorkspace
    • allow to specify a plot prefix and a different temporary directory
    • allow to override config file parameters by specifying a json dictionary on the command line
• Bs2DsPi configuration file that are needed for the automated FT-Calibration, 2018 datasets are included
• minor code formatting and some minimal refactoring of configuration parsing