Stack() doesn't mask array when a file path is provided

[lwasser]

Describe the bug
This is really weird. in the week 6 demo notebook, when you provide an output path for stack it does NOT return a masked array. But when you do provide a path it DOES return a masked array. This is not the expected behavior.

To Reproduce
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3. Scroll down to '....'
4. See error

Expected behavior
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**What Operating System Are you Running? **

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[nkorinek]

This issue was confusing me, so I made a gist to describe why I am confused, here: https://gist.github.com/nkorinek/655d590da99b5f26dca8b02208599aea. @lwasser let me know if the gist makes sense? I'll put in the fix suggested if you give the go ahead

[lwasser]

please go ahead and implement this fix!

[lwasser]

closing this as it was addressed in #525