Tendency variable has incorrect units

In `schemes/pumas/micro_pumas_ccpp_dimensions_pre.meta`, the variable `pumas_frzimm` has the standard name:

> tendency_of_pumas_cloud_liquid_droplet_number_concentration_due_to_immersion_freezing

However, the units are listed as `cm-3`, which violates the standard name convention that all `tendency_of_X` variables must have a `s-1` in the listed units. Thus either the provided units need to change, or the standard name needs to change to remove the `tendency_of_` prefix.