Add new observation formulas

[mvertens]

Summary:

When ADF compares the model against observations, it needs the obs data for each variable — but that data isn't always sitting in the obs file ready to use. Sometimes the file has the ingredients but not the final field (it stores the two kinds of rain, PRECC and PRECL, but not total rain PRECT). Sometimes the variable is stored under a different name, or in different units than the model uses. Until now, ADF could only rename a variable and multiply it by a fixed
number, which wasn't enough to cover these cases.

This PR lets you write a short formula that tells ADF how to construct the observational variable from whatever the file actually contains. For example:

• PRECT = PRECC + PRECL — add the pieces together
• ALBEDO = (solar_mon − fsnt) / solar_mon — a genuine calculation
• TGCLDLWP = TGCLDWP × 1000 — rename and convert units at once

What you put in a variable's settings

Two new settings do the work, and one existing setting now has a clearer, narrower role:

Setting	What it's for	When you use it
obs_var_name	The name the variable is stored under in the obs file	Only when the obs file stores the field directly but calls it something different from ADF (e.g. ADF's AODVIS is AOD_550 in the file). Skip it when the name already matches, and skip it for derived variables — there's no single stored field to point at.
obs_derivable_from (new)	The list of input variables, named as they appear in the obs file, that the formula needs	When the variable has to be built from other fields in the file
obs_derivation_formula (new)	The formula itself, written in terms of those input names	Alongside obs_derivable_from — the two go together

So it's an either/or: a variable that's stored directly may just need obs_var_name (if the name differs); a variable that's computed uses obs_derivable_from + obs_derivation_formula and leaves obs_var_name out
entirely. A quick example of the derived case:

ALBEDO:
  obs_file: "CERES_EBAF_Ed4.1_2001-2020.nc"
  obs_name: "CERES-EBAF"
  obs_derivable_from: ['solar_mon', 'fsnt']          # what's actually in the file
  obs_derivation_formula: "(solar_mon - fsnt) / solar_mon"

[TomasTorsvik]

@mvertens - thanks for the OBS fix, looks fine to me. Is this update also shared with the NCAR version of ADF?

[mvertens]

@TomasTorsvik - thanks for reviewing this.

We are up to date with the latest version of the NCAR repository. However, I have added a lot of new features that we want and that have not been shared with NCAR. At this point I have another PR that is currently in draft form that I will test once this PR is in. Then I'd for us to have the changes that @adagj will be bringing in that she is using. And at that point I'd like to do a PR back to NCAR.