Add an extra dimension if the dimension does not exist for cat

For `cat`, there should be an option to add an extra dimension with a specified vector for the dimension. This make it possible to do something like splitting by seasons across time, taking time average, and concatenating along a "fake" time dimension.

Start by locating the `cat` implementation and its existing dimension-handling tests or entry points. Determine how an extra dimension and its specified vector should be represented, then verify that seasonal splitting, time averaging, and concatenation along the synthetic time dimension work as described.

Written by the indexing model from the issue text.