Provide suitable string format for TGraph constustructors

[linev]

Follow up for #20368

There are three classes - TGraph, TGraphErrors and TGraphAsymmErrors - which can be created from text file.
But default format value fit only with "CSV" file format which has "," as delimiter.
So when reading other formats like TXT or TSV, one need to create matching format string.

While all these classes have SaveAs method to store graph objects in the text file with CSV, TSV and TXT extension,
just provide suitable format string for such files extension when reading file. So one can do:

TGraphErrors gr;
gr.SetPoint(1, 3., 3.);
gr.SaveAs("file.tsv", "asroot");

auto gr2 = new TGraphErrors("file.tsv", "");

Use feature in SaveAs testing

[github-actions]

Test Results

    21 files      21 suites   3d 23h 52m 2s ⏱️
 3 749 tests  3 749 ✅ 0 💤 0 ❌
76 820 runs  76 820 ✅ 0 💤 0 ❌

Results for commit e86c3d3.