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
```

Start by locating the OutputVar type and its existing dimension-manipulation functions. Read the linked ClimaCalibrate.jl issue for the calibration use case and check how dimensions and values are represented. Done means a public function can insert the requested singleton dimension into an OutputVar, with tests covering the example behavior.

Written by the indexing model from the issue text.