obs_impact_tool documentation improvements

[c-merchant]

Description:

I added "Filter Use" and "Building" sections and details to obs_impact_tool docs and deleted typo in assim_tools_mod.rst.

Fixes issue

PR intends to fix issue #571

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Documentation changes needed?

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Tests

I ran the standard ./quickbuild.sh, ./perfect_model_obs, and ./filter for Lorenz-63 to make sure no bugs were introduced.

Checklist for merging

• Updated changelog entry
• Documentation updated
• Update conf.py

Checklist for release

• Merge into main
• Create release from the main branch with appropriate tag
• Delete feature-branch

Testing Datasets

• Dataset needed for testing available upon request
• Dataset download instructions included
• No dataset needed

[nancycollins]

Looks good. I suggested a minor change to the wording to be more clear about the tool vs output from the tool.

[hkershaw-brown]

This is good start Charlotte.
At the moment this documentation is describing what the code does, rather than how to use it.

The user wants to restrict the impact of some observations on some quantities.
How do they do that?

• build obs_impact_tool
• create a cross_correlations.txt where you define the impacts.
• Run obs_impact_tool to create control_impact_runtime.txt
• Use this contrpol_impact_runtime.txt file in your assimilation by setting:
  &assim_tools_nml
  adjust_obs_impact = .true.
  obs_impact_filename = 'control_impact_runtime.txt'
  /

Let's chat today about next steps. This documentation page (both the existing and the changes) is a good candidate for taking a look at with the documentation style guide short summary
https://developers.google.com/style.

Cheers,
Helen

[hkershaw-brown]

@c-merchant approved changes