Remove special field EVENT, add parameter event_variables to FunTuple

Closes DaVinci#43 (closed)

With this merge request event variables have their own parameter in the FUnTuple function instead of using a special key in the variables dict.

evt_variables = FunctorCollection({
        "RUNNUMBER":
        F.RUNNUMBER(odin),
        "EVENTNUMBER":
        F.EVENTNUMBER(odin),
        "Hlt2_TCK":
        F.TCK(hlt2_dec),
        "Sel":
        F.DECISIONS(Lines=hlt2_lines, DecReports=hlt2_dec),
})
variables = {
    "D0": d0_variables,
    "Kminus": daughter_variables,
    "piplus": daughter_variables}

my_tuple = Funtuple(
        name="Tuple",
        tuple_name="DecayTree",
        fields=fields,
        variables=variables,
        event_variables=evt_variables,
        inputs=d02kpi_data)

Requires DaVinci!669 (merged)