Feature/diagnostics level select

[elynnwu]

Purpose

This PR adds the capability of saving a vertical slice using z_select in the diagnostics config.

The expected syntax is as follows:

diagnostics_config:
  path: output
  output_format: netcdf
  z_select:
    - level: 1
       names:
         - ua
         - va
    - level: 10
       names:
         - pt  

Code changes:

• diagnostics.py: added ZSelect class for storing selected vertical slice and the variables to be saved
  • fix column integral mistake, need to multiple by 1/gravity

Checklist

Before submitting this PR, please make sure:

• You have followed the coding standards guidelines established at Code Review Checklist.
• Docstrings and type hints are added to new and updated routines, as appropriate
• All relevant documentation has been updated or added (e.g. README, CONTRIBUTING docs)
• For each public change and fix in pace-util, HISTORY has been updated
• Unit tests are added or updated for non-stencil code changes

[twicki]

just a few questions

[twicki]

Did we decide to re-think the layout in the config in a later PR? Or did you and @mcgibbon come to this as the final look?

[elynnwu]

We converged to this layout because it is saved as a separate variable instead of as part of the snapshot.

[twicki]

technically we select and don't slice here - not super relevant as selecting is a 1-sized slice, but if we actually wan to implement both we might want to rename it here already?

[elynnwu]

Changed it to select data.