trace: Fix base behaviour for TraceView/Trace

Due to the __getattr__ magic we do in TraceView, doing

    view = TraceView(...)
    df = view.analysis(...)

    view.add_events_deltas(df)

would end up calling `Trace.add_events_deltas`, which uses the whole
trace's duration for the last delta, which is wrong.

Fix that by letting having add_events_deltas() in the base trace class
shared by Trace and TraceView - while at it, renamed it TraceBase.

Reported-by: Quentin Perret <quentin.perret@arm.com>