Extract all correlation functions if not specified

I think it would make sense if the CorrFunctionGetter extracts all correlation functions in case no correlation functions are specified. I propose to change

class CorrFuncEnergyDataManager(DataManager):
def __init__(self, db_name: str, cf_names: List[str],
                 tab_name: str) -> None:

to

class CorrFuncEnergyDataManager(DataManager):
def __init__(self, db_name: str, tab_name: str, 
cf_names: Optional[List[str]] = None) -> None:

and similar elsewhere. Thus, the default behavior of all data managers is to extract all correlation functions in the database. This will make the data managers much easier to use as standalone objects.