Use CA arg parser in RunTrigCostAnalysis
While trying to use RunTrigCostAnalysis.py on some recent data, I noticed that it's using the plain ArgParse parser and not the Athena one, which is better integrated with the flags and doesn't require interpretation of standard Athena arguments. So this MR switches over to the AthOptionParser solution. The one difference that might break scripts and whatnot is that we consequently use --evtMax rather than --maxEvents. Apart from that, --loglevel gets propagated to MessageSvc rather than just the TrigCostAnalysis components -- but one can already customise components' OutputLevel by other means.