Add getters for dimension names and values

[ph-kev]

When using ClimaAnalysis, you might need to do something like keys(var.dims)) to access the names of the dimensions. It would be convenient to provide ClimaAnalysis.dimnames as a getter for the names of the dimensions.

Similarly, it might also be convenient to provide the function ClimaAnalysis.dimvalues to get the values of a dimension when passed a name of a dimension.

[ph-kev]

In #390, a getter for the dimension names are added. It is unclear to me whether a getter for the dimension values should be added since that functionality is accomplished via longitudes, latitudes, times, and the other variants of these functions.