Add land mask to sterodynamic component

[Hemant Khatri (hmkhatri)]

This pull request add land mask. It reads in all available land masks and applies the union of those land masks to all models. The union is required because some model have weird land mask or not have at all.

Advantage of this approach is that only ocean grid points present in all models are considered. The downside is that some of the grid points near the continental boundaries are lost (see figure).

The sterodynamic output has nans over land. The code does not affect other other components, which have data over the whole globe.

Greg Munday (@gregrmunday) Could you please review it?

[Hemant Khatri (hmkhatri)]

Here are slops and mask from all models. GISS-E3-1-H does not have mask values and INM-CM4-8 doe not have land mask over some parts of Russia. These are not major issues though and the figure has only been added here to provide a full picture.

[Greg Munday (gregrmunday)]

Thanks Hemant, looks good to me! Just wanted to double check that you're happy with the padding/longitudinal alignment of the masks?

Can see in the figure up top that the funnel-y artefacts from the ocean grids aren't in the same place - is this expected?