ENH: Support multiline header fields in TCK

[effigies]

@mattcieslak's initial PR was lost in the refactor by @anibalsolon in #1140. This PR takes the data file shared by @valeriejill, with just the first streamline (zeroed out) to confirm that a header that was failing to be read now can be. I've also added an extra command_history line later in the header to ensure that we aggregate duplicate keys instead of overwrite, in response to #957 (comment) by @thijsdhollander.

@valeriejill can you confirm that we have permission to publish this file as part of our test data? If not, I think I've learned enough about working with the format to make my own.

@anibalsolon @MarcCote I would appreciate code reviews from you.

Closes #957.

[codecov]

Codecov Report

Base: 92.39% // Head: 92.39% // No change to project coverage 👍

Coverage data is based on head (a60d2b7) compared to base (a60d2b7).
Patch has no changes to coverable lines.

❗ Current head a60d2b7 differs from pull request most recent head 95df42e. Consider uploading reports for the commit 95df42e to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1175   +/-   ##
=======================================
  Coverage   92.39%   92.39%           
=======================================
  Files          97       97           
  Lines       12264    12264           
  Branches     2525     2525           
=======================================
  Hits        11331    11331           
  Misses        613      613           
  Partials      320      320           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[effigies]

Any chance of a review? I'm comfortable with the test coverage and will plan to merge tomorrow unless someone asks me to wait.

[mattcieslak]

This looks good to me, thanks for revisiting it @effigies!

[effigies]

Thanks for the review! And thanks for your patience with the... sporadic activity here.

[MarcCote]

Thanks @effigies for maintaining this code :).

[anibalsolon]

Getting back-
Nice! Hopefully there's no colon in new lines 🤷🏻‍♂️ it would be great if new lines were escaped (\n) when mrtrix writes them