Fix the standard deviation in the sea ice contour plots.

[dabail10]

Description of changes:

This addresses issue #402. Turns out that I needed nanstd. Also tweak the colormaps and mask.

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edit [MNL]: removed superfluous checkbox from description section

[mnlevy1981]

Looks good! I know np.nanstd() fixed the issue with vmin and vmax, and I like how the arguments were cleaned up (listing vmin before vmax and passing [constant]*field_std instead of "field_std*[constant])

[dabail10]

@duvivier Can you take a quick look at this?

[duvivier]

Looks good to me!