[FIX] File: Construct unique column names.

[thocevar]

Issue

Fixes #4381

Description of changes

File readers constructing a data table now rely on get_unique_names_duplicates from data.util instead of using their own similar logic.

Fixed get_unique_names_duplicates for cases where appending an index collides with an exiting name (['x', 'x', 'x (2)'] now returns ['x (1)', 'x (3)', 'x (2)'].

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

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The diff coverage is 100%.

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