If `attributes["units"]` is not a string, we cannot `uparse` it. This happens
all the times we return a new `OutputVar` from one we have already processed.

This commit adds a check that accounts for this case too. In this case, we just
have to return the value.

This commit adds support for applying land/sea masks to GeoMakie plots. The
masks are obtained directly from GeoMakie as a collection of polygons. For the
ocean mask, we use bathymetry data at 0 elevation. Data is provided by
`NaturalEarth.jl`. (I had to bump the minimum compat for GeoMakie to 0.7 because
support `NaturalEarth.jl` was introduced in that release.)

This new option is only aesthetics: it does change the underlying data (we
simply overplot something else). This also means that the colorbar will include
values under the mask. Applying the mask directly to the data is left for future
work.

Thanks @haakon-e for help with this feature.