Add variable dimensions to available_diags file

[jkrasting]

The output of the current available_diags file lacks the dimensions of each diagnostic.

This PR includes a commit that adds the appropriate horizontal and vertical dimension names to the entries listed in the available_diags file. Scalar diagnostics are denoted with a "scalar" string.

Example of an entry generated with the new code:

"temp"  [Used] (CMOR equivalent is "thetao")
    ! modules: {ocean_model,ocean_model_z,ocean_model_rho2,ocean_model_rho0,ocean_model_d2,ocean_model_z_d2,ocean_model_rho2_d2,ocean_model_rho0_d2}
    ! dimensions: yh, xh, zl
    ! long_name: Potential Temperature
    ! units: degC
    ! cell_methods: xh:mean yh:mean zl:mean area:mean
    ! variants: {temp,temp_xyave,thetao,thetao_xyave}

[Hallberg-NOAA]

Overall I like what is done here. I have two questions, though, to consider in perhaps minorly revising this code:

1. Why is there a space before the comma on lines 2389-2392 and similarly on lines 3133-3136 and not after? (I.e., why is the end of this lines " ,yh, xh" instead of ", yh, xh"?
2. Why is the order of dimensions here given as (for example) "yh, xh, zl" instead of the "xh, yh, zl" that might seem more natural?

[marshallward]

Apologies for moving the goalposts, but if we're considering revisions to available_diags, then we could also convert it to YAML:

temp:
    used: True
    cmor_equivalent: thetao
    modules: [ocean_model, ocean_model_z, ocean_model_rho2, ocean_model_rho0, ocean_model_d2, ocean_model_z_d2, ocean_model_rho2_d2, ocean_model_rho0_d2]
    dimensions: [yh, xh, zl]
    long_name: Potential Temperature
    units: degC
    cell_methods: [xh:mean, yh:mean, zl:mean, area:mean]
    variants: [temp, temp_xyave, thetao, thetao_xyave]

This would make postprocessing easier for many people.