Allow plotting utilest signals

• Add may_use_events() decorator to supplement requires_events() and requires_one_event_of()
• Add LoadTrackingAnalysis.df_(cpus|task)_signal to replace df_(cpus|task)_signals()
• Allow plotting util_est_enqueued signals in LoadTrackingAnalysis.plot_(cpus|task)_signals
• Allow selecting signals to plot in LoadTrackingAnalysis.plot_cpus_signals to mirror the task variant of the method

EDIT:

• Add utils.deprecate() decorator and treat deprecation as errors in lisa and warnings in __main__