More cosmetic and usability changes

[peremato]

Added extra usability functions:

• GetObject(file::Union{ROOT.TDirectoryFile, CxxPtr{<:ROOT.TDirectoryFile}}, name::AbstractString)
• Base.getproperty(file::Union{ROOT.TDirectoryFile, CxxPtr{<:ROOT.TDirectoryFile}}, sym::Symbol)

This allows to get all sort of objects from a TFile

julia> f = ROOT.TFile!Open("demo_ROOT_out.root");

julia> @test GetObject(f,"h") isa ROOT.TH1
Test Passed

julia> f = ROOT.TFile!Open("test1.root")
CxxWrap.CxxWrapCore.CxxPtr{ROOT.TFile}(Ptr{ROOT.TFile} @0x000000012c833d20)

julia> @test f.tree isa ROOT.TTree
Test Passed

[grasph]

@peremato can you review the changes I made and tell me if they are ok for you?