Add `keepdim` keyword argument for reduction functions

Related to https://github.com/CliMA/ClimaAnalysis.jl/issues/279, for functions that perform reductions, there should be a keyword argument that keep the dimension (can be at the first element of the dimension).

This is needed for calibration (see https://github.com/CliMA/ClimaCalibrate.jl/issues/267) as there is an implicit assumption that a time dimension is required. Alternatively, this solution can also be tackled in ClimaCalibrate by resolving the issue that is linked.

Start by reviewing the functions that perform reductions and the requirements in the linked ClimaAnalysis issue 279 and ClimaCalibrate issue 267. Determine whether the change belongs in ClimaAnalysis.jl or ClimaCalibrate.jl, then verify that the chosen solution preserves the required time dimension for calibration.

Written by the indexing model from the issue text.