Feature request: Better obs_impact_tool documentation

[josephCMY]

Use case

Using obs_impact_tool with WRF-DART to turn off model increments for certain variables

Is your feature request related to a problem?

The documentation for obs_impact_tool only cover how to use the obs_impact_tool program. It is missing a description of how to use the output with filter.

Describe your preferred solution

Add a blurb about what namelist options to specify in input.nml when calling filter.

[hkershaw-brown]

assim_tools_nml namelist options required o use the obs impact code during filter"

  adjust_obs_impact               = .true.
  obs_impact_filename             = 'control_impact_runtime.txt'

obs_impact_tool is used to create the 'control_impact_runtime.txt' file:
https://docs.dart.ucar.edu/en/latest/assimilation_code/programs/obs_impact_tool/obs_impact_tool.html

[hkershaw-brown]

add to quickbuild.sh

[hkershaw-brown]

fixed in #655