Add _FillValue attribute to most fields in MALI Registry

[hollyhan]

This PR adds cf-compliant default_value and missing_value attributes to MALI registry files.

[xylar]

@hollyhan, the other part of handling fill values will be creating an iceCellMask (or similar) integer variable of 0s and 1s that can be used with the work @cbegeman did in E3SM-Project#5154. This should presumably be a separate PR.

I'm wondering if it might make sense to work a few variables at a time rather than adding fill values to all variables at once. I think that's the strategy we ended up preferring by the end of the May hackathon.