add directory for applying perturbation

[zhichang-guo]

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Describe your changes

Summarise all code changes included in PR: Add directory and codes for applying perturbation to vegetation fraction

List any associated PRs in the submodules. None

Issue ticket number and link

List the git Issue that this PR addresses: https://github.com/noaa-psd/land-offline_workflow/issues/3

Test output

Is this PR expected to pass the DA_IMS_test (ie., does it change the output)? Yes

Does it pass the DA_IMS_test? Yes

If changes to the test results are expected, what are these changes? Provide a link to the output directory when running the test:

Checklist before requesting a review

• [X] My branch being merged is up to date with the latest develop.
• [X] I have performed a self-review of my code by examining the differences that will be merged.
• [X] I have not made any unnecessary code changes / changed any default behavior.
• [X] My code passes the DA_IMS_test, or differences can be explained.

[ClaraDraper-NOAA]

@zhichang-guo Can you please resolve the conflicts in this PR so that we can get it merged?

[ClaraDraper-NOAA]

@zhichang-guo I think so. You also closed the PR though - can you please re-open it? ( I don't seem to be able to).

[zhichang-guo]

After I resolved the conflicts, the PR has been automatically closed. So reopen it. I think it's ready to be merged.

[ClaraDraper-NOAA]

Thanks @zhichang-guo I'll have a look later today.

[ClaraDraper-NOAA]

Thanks @zhichang-guo . I've merged it.