MultiModal metric threshold override

[FlorianDeconinck]

Description
Add threshold overrides to the multimodal metric using the following yaml schema

ComputeUwshcu:
  - backend: dace:cpu
    multimodal:
      mmr_relative_fraction: 1e-2
      mmr_absolute_eps: 1e-10 
      mmr_ulp_threshold: 10

How Has This Been Tested?
Locally on GEOS physics code

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[oelbert]

At this point it's probably useful to document the error thresholding and reporting behavior somewhere, like to explain what ulp_threshold means or why some tests use the threshold yamls vs setting self._max_error

[FlorianDeconinck]

At this point it's probably useful to document the error thresholding and reporting behavior somewhere, like to explain what ulp_threshold means or why some tests use the threshold yamls vs setting self._max_error

There's some instruction in pyFV3. Let me unearth those, complete them and moved them here

[FlorianDeconinck]

Ping @oelbert / @fmalatino: this is blocking us

[fmalatino]

lgtm

[fmalatino]

lgtm