CGNS format for FFD boxes

[MicK7]

Proposed Changes

• This PR proposes to add CGNS format as a possible writing format for FFD box

Related Work

• This should address Implement/store FFD box info in CGNS mesh files #148

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[pcarruscag]

Hi @MicK7,
Thank you for the feature, it looks ok to me. But cleaning the indentation may cause a lot of merge issues to others, so I will give it time to see if anyone has issues.
Cheers,
Pedro

[economon]

LGTM. Indentation is ok for me, but let's see if anyone else has a concern.

This is a very nice contribution @MicK7 - thanks! Btw, since you clearly have some experience with the CGNS library, we could certainly use help with a few other CGNS-related items. Let me know if you are interested, and I can share more.

[pcarruscag]

LGTM. Thank you again for the contribution @MicK7.