groot: add read support for TCanvas

[sbinet]

No description provided.

[codecov-commenter]

Codecov Report

Attention: 88 lines in your changes are missing coverage. Please review.

Comparison is base (e73dd33) 72.88% compared to head (8af6fdd) 73.51%.

Files	Patch %	Lines
groot/rpad/pad.go	81.25%	18 Missing and 6 partials ⚠️
groot/rpad/canvas.go	73.91%	13 Missing and 5 partials ⚠️
groot/rpad/attcanvas.go	51.72%	11 Missing and 3 partials ⚠️
groot/riofs/walk.go	52.17%	11 Missing ⚠️
groot/rcmd/dump.go	40.00%	6 Missing and 3 partials ⚠️
groot/rbase/vpad.go	73.07%	5 Missing and 2 partials ⚠️
groot/rcmd/ls.go	91.66%	2 Missing and 1 partial ⚠️
groot/rbase/qobject.go	83.33%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1002      +/-   ##
==========================================
+ Coverage   72.88%   73.51%   +0.63%     
==========================================
  Files         420      426       +6     
  Lines       63821    65659    +1838     
==========================================
+ Hits        46514    48270    +1756     
- Misses      14913    14976      +63     
- Partials     2394     2413      +19     

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