Add function to add singleton dimension

[ph-kev]

For calibration (see CliMA/ClimaCalibrate.jl#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