CSV Writer Allow Varying Number of Columns

[oleibman]

Supersedes PR #1415 by @AndrewMonty, which went stale in May 2020, and which is not directly usable due to changes between now and then. Fix #1414, which also went stale; I will remove the stale status and reopen the issue pending the merging of this PR.

Add an option to CSV Writer so that it writes the cells for a row only through the highest data column used in the row, rather than through the highest data column used in the worksheet.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • [] Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.