Add getters for dimension names and values

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.

Start by examining the existing dimension access represented by `keys(var.dims)` and the surrounding ClimaAnalysis API. Confirm how dimension names and values are currently retrieved, then check that the proposed `ClimaAnalysis.dimnames` getter exposes names and `ClimaAnalysis.dimvalues` returns values for a dimension name. Done means both access patterns are available and covered by appropriate tests.

Written by the indexing model from the issue text.