TLeafG Support

[tamasgal]

64 bits integer were supported by Root long before 6.24 (TLeafL which can be UInt64 or Int64 depending on the fIsUnsigned field) but 6.24 introduced the support for it in "simple leaves" (whatever that means) by adding a new class called TLeafG.

This PR adds support for it.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.62%. Comparing base (e766112) to head (ec48568).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/streamers.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #379      +/-   ##
==========================================
+ Coverage   83.59%   83.62%   +0.03%     
==========================================
  Files          21       21              
  Lines        3085     3097      +12     
==========================================
+ Hits         2579     2590      +11     
- Misses        506      507       +1     

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