Define runPeriod in a better (and backward compatible) way

Following a suggestion from Elena Vernazza (I can't tag her somehow), this MR moves the runPeriod attribute from a Dataset parameter to a Config parameter. This MR alone should have no impact on any previous installation of the NBA framework. The updates of this MR can be pulled by running the following command cmt_pip_install git+https://gitlab.cern.ch/cms-phys-ciemat/analysis_tools.git

The only required change should be for @emartinv (who to my knowledge is the only one that used the implementation of this previous MR) to re-run the updated run3_dataset_config_initializer.py and add the runPeriod attribute in the configurations definitions (i.e. here, and here).