Fix bug creating QTable with units arg set

[taldcroft]

Description

This pull request is to fix a problem when a QTable is created and one or more columns are Quantity objects and units is provided to update the Quantity units.

Previously the code was always setting the column units attribute, but this does not work for Quantity. Instead in that case update the column unit in-place using the <<= operator.

Fixes #14336

[github-actions]

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[mhvk]

Short & sweet!