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]

Thanks Pere for this nice PR. It's great you find how to implement a generic get function to retrieve objects from the TFile. I've added some comments about improvements.

[grasph]

I don't get the purpose of the change. We don't necessarily know in advance the maximum size of a collection.

The example was showing how to do in case of variable length collection.

I think the original example should be kept.

[grasph]

We need to return 'c' to keep a reference to the TCanvas, which otherwise can be garbage collected at any time.

Why the call to 'Draw()'? Call to 'Fit' already triggers a draw, at least on Linux. Does the behaviour differ on mac (I don't have the possibility to test it)?

[grasph]

Restored original code. Confirmation that the canvas is plotted on MacOS is needed.

[grasph]

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