Add function to add singleton dimension

For calibration (see https://github.com/CliMA/ClimaCalibrate.jl/issues/257), you may want to do a time average, but the time dimension is  gone in this case. To help with this, it would be nice to have a function that insert a singleton dimension to a `OutputVar`.

For example,
```
function insert_singleton_dimension(var::OutputVar, dim_name, dim_value)
       # implementation here
end
``` 