Add Julia port of atan2 from Openlibm and add some tests.

[pkofod]

Not tested for performance yet.

[pkofod]

Does anyone have an idea what the following test failure is about?

    FAILURE
Error in testset statistics:
Error During Test
  Got an exception of type LoadError outside of a @test
  LoadError: return type RowVector{Float64,Array{Float64,1}} does not match inferred return type Any
  Stacktrace:
   [1] include_relative(::Module, ::String) at ./loading.jl:464
   [2] eval(::Module, ::Any) at ./boot.jl:291
   [3] checked_add at ./checked.jl:170 [inlined]
   [4] length at ./range.jl:408 [inlined]
   [5] getindex at ./array.jl:770 [inlined]
   [6] choosetests(::Array{String,1}) at /usr/home/julia/worker/11rel-amd64/build/test/choosetests.jl:57
   [7] vect at ./array.jl:96 [inlined]
   [8] choosetests(::Array{String,1}) at /usr/home/julia/worker/11rel-amd64/build/test/choosetests.jl:151
   [9] choosetests(::Array{String,1}) at /usr/home/julia/worker/11rel-amd64/build/test/choosetests.jl:146
   [10] choosetests(::Array{String,1}) at /usr/home/julia/worker/11rel-amd64/build/test/choosetests.jl:148
   [11] choosetests(::Array{String,1}) at /usr/home/julia/worker/11rel-amd64/build/test/choosetests.jl:139
  while loading /usr/home/julia/worker/11rel-amd64/build/test/statistics.jl, in expression starting on line 177

[fredrikekre]

#23371

[pkofod]

Travis has some test failures. They don't look nice, but I doubt they are related. #23796

[ararslan]

@nanosoldier runbenchmarks("scalar", vs=":master")

[ararslan]

Had to kick the server. @nanosoldier runbenchmarks("scalar", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @ararslan

[pkofod]

Nanosoldier seems to be ok (? @simonbyrne )