Fix mask in land variables for CCSM4 (fixes #578)

[zklaus]

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Closes #578

[schlunma]

Code looks good, didn't test though. If you provide a small recipe I can run a quick test on DKRZ if you like.

[schlunma]

Successfully tested on DKRZ, plots look okay (there is a weird line at lon=0, but I guess that is not related to this PR).

[zklaus]

Cheers 🍻