Add methods in preparation for MRH implementation

In preparation of including the MRH workflows to the Automation Framework, this PR adds a few useful methods:

• a new getRunEvents methos is added to the QueryDBS class API
• a new T0GeneralAPI is added to ecalautoctrl/tier0_api.py in order to allow direct access to T0 API via https queries, for the moment it contains only two methods:
  • get_prompt_gt to retrieve the Prompt GT (this method can be used e.g. in processing_ctrl.py)
  • get_MRH_values to retrieve a few values of the Express jobs useful for MRH checks
• a new decorator process_by_nevents to aggregate runs in groups based on a target number of events is added to ecalautoctrl/JobHandlers.py