introduce a fixed time window mode.

An example:

#!/usr/bin/env python 
import os
from Gaudi.Configuration import *

from Configurables import k4DataSvc
dsvc = k4DataSvc("EventDataSvc", input="simsignal_v01.root")

from Configurables import PodioInput
podioinput = PodioInput("PodioReader", collections=[
    "MCParticle",
    "VXDCollection",
    "ITKBarrelCollection",
    "ITKEndcapCollection",
    "TPCCollection",
    "OTKBarrelCollection",
    "OTKEndcapCollection",
    "EcalBarrelCollection", "EcalBarrelContributionCollection",
    "EcalEndcapsCollection", "EcalEndcapsContributionCollection",
    "EcalEndcapRingCollection", "EcalEndcapRingContributionCollection",
    "HcalBarrelCollection", "HcalBarrelContributionCollection",
    "HcalEndcapsCollection", "HcalEndcapsContributionCollection",
    "HcalEndcapRingCollection", "HcalEndcapRingContributionCollection",
    "MuonBarrelCollection",
    "MuonEndcapCollection"
])

from Configurables import DetSimMixingAlg
evtmixing = DetSimMixingAlg("EvtMixingAlg")
# evtmixing.BackgroundRates = {"testing": 1.0e6} 
# evtmixing.BackgroundFileLists = {"testing": ["test.root"]} 

evtmixing.BackgroundRates = {"testing": -3460}
evtmixing.BackgroundFileLists = {"testing": ["test-bgsimdose.root"]}

# output 
from Configurables import PodioOutput
out = PodioOutput("outputalg")
out.filename = "evtmix_v01.root"
out.outputCommands = ["keep *"]

from Configurables import ApplicationMgr
ApplicationMgr().TopAlg += [podioinput, evtmixing, out]
ApplicationMgr().ExtSvc += [dsvc]
ApplicationMgr().EvtSel = "NONE"
ApplicationMgr().EvtMax = 10
ApplicationMgr().OutputLevel = INFO