+Add and use optional conversion argument to register_field in MOM_io_file

[Hallberg-NOAA]

This PR consists of three commits, the first of which adds the ability to rescale output variables that are written by the MOM_io_file routines analogously to what is already available for variables that are written via MOM_write_field() or via MOM_diag_mediator(). The second commit uses this new capability in write_energy() and revises the internal calculations in write_energy() to work almost entirely with rescaled variables. The third commit corrects 4 conversion arguments to var_desc() calls now that they might actually be used.

The specific changes include the addition of a new optional conversion argument to register_field(), code to rescale variables in 10 write_field() routines, changes to modify_vardesc() and query_vardesc() to properly store and retrieve conversion factors, and the use of these new capabilities in ocean_register_diag() and write_energy().

All answers and output are bitwise identical, but there are new optional arguments to publicly visible types. The specific commits in this PR include:

• e2be48235 Fix 4 conversion arguments to var_desc calls
• cb2f49094 Specify conversion factors in write_energy
• c3da6766e +Add optional conversion argument to register_field

[marshallward]

I can't recall if MOM_field was meant to be extended, but I think it's the correct choice. The call write_field() inside of write_field_* is going into the backend, which should not do any rescaling.

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The only comment I have is that implicit allocation could simplify some of these functions.

The unscaled_field(..) variables could be removed here. For this block:

    allocate(unscaled_field, source=field)
    unscaled_field(:,:,:) = field_md%conversion * field(:,:,:)
    call write_field(handle%handle_infra, field_infra, MOM_domain, unscaled_field, &
        tstamp=tstamp, tile_count=tile_count, fill_value=fill_value)
    deallocate(unscaled_field)

you could use implicit allocation and write it as

    unscaled_field(:,:,:) = field_md%conversion * field(:,:,:)
    call write_field(handle%handle_infra, field_infra, MOM_domain, unscaled_field, &
        tstamp=tstamp, tile_count=tile_count, fill_value=fill_value)

Or you just put in the argument

    call write_field(handle%handle_infra, field_infra, MOM_domain, field_md%conversion*field, &
        tstamp=tstamp, tile_count=tile_count, fill_value=fill_value)

All of these should be equivalent, so I will tenatively approve this and let you decide if you want to make the adjustment.