Rename CCPP_interstitial to GFDL_interstitial (#181)

[JiliDong-NOAA]

Description

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Fixes # (issue)

How Has This Been Tested?

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Checklist:

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• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules