Fix Append Transformation Edge Cases

[miles-grant-ibigroup]

Checklist

• Appropriate branch selected (all PRs must first be merged to dev before they can be merged to master)
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Description

The append transform wasn't handling newlines correctly. Newlines in the middle of a file would cause datatools to stop processing right there, causing problems.

I'm not entirely sold on the solution I've found here of creating a second file to store the non-newlined text contents. My thinking was this was more stable than relying on memory (imagine an >100mb text file, which happens!)

[br648]

Just some minor items to address. I think given how large these files can get, this is probably the best approach. Unit test fails on Windows OS. I may address this one day.

[josh-willis-arcadis]

Looks good!