JunyanHuo

NDCube.fill method

5

## PR Description This pull request aims to resolve the issue created by @DanRyanIrish , https://github.com/sunpy/ndcube/issues/828 .

Added new features to the ndcube.__add__ method

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## PR Description This PR aims to fix issue #734 created by @DanRyanIrish . write down the scenarios. (list, to see what has been covered) ### Testing scenarios without mask...