Constructing OutputVar from a function

It may be helpful to provide a constructor for making `OutputVar` using a function.

For example, when computing the covariance matrix for calibration, you can add latitude weights by computing 1 / lat and flattening the data. See the example below of this.

```
function lat_weight_mat(output_var::OutputVar)
lon_size, lat_size = size(output_var.data)[1:2]
lat_weights = zeros(Float32, lon_size, lat_size)
for i in 1:lon_size
for j in 1:lat_size
lat_weights[i, j] = 1.0f0 ./ max(cosd(ClimaAnalysis.latitudes(output_var)[j]), 0.1)
end
end
return lat_weights
end
```

It would be helpful to provide a function that allow the user to pass in their own function to construct `OutputVar`s from.

Start by locating the OutputVar definition and any existing constructors, then inspect how the latitude-weighting example would supply a function and its inputs. Done means a documented function-based construction path exists for OutputVar and its behavior is covered by appropriate tests.

Written by the indexing model from the issue text.