Don't override col_cls in DynamicTable.add_column

[rly]

Motivation

Issue: If a user extends DynamicTableRegion or EnumData and tries to create a column with that subclass, using the table or enum keyword args, their subclass argument gets replaced with the base DynamicTableRegion or EnumData classes.
Solution: Use the passed col_cls. If not provided, then use the base DynamicTableRegion or EnumData classes.

TODO: Add a quick test

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[mavaylon1]

Will this be in by next milestone?

[VisLab]

Please let us know when it gets in. We are relying on it for implementation of ndx-hed.