FR: merge! which initializes tree_point from passed timer

[mfherbst]

When doing timings in multi-threaded regions currently using the merge! function requires one to pass exactly the path at which the timings are supposed to be stored. Especially for larger nested timings where moreover the nesting might depend on the call hierarchy this is not convenient.

Apart from the breaking change is there any reason not to have the definition

function Base.merge!(self::TimerOutput, others::TimerOutput...; tree_point = [t.name for t in self.timer_stack])
...
end

such that this pattern

const to = TimerOutput()

function workfunction()
    Threads.@threads for i = 1:100
       tlocal = TimerOutput()
       @timeit tlocal "work" sleep(2)
       merge!(to, tlocal)
    end
end

@timeit to "outer" begin
    @timeit to "inner" begin
        workfunction()
   end
end

would automatically accumulate the timings in the path outer/inner/work?