Dispersal changes

[CannedIce]

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What Makes a Good Pull Request

Moved growth form out to branch
Implemented and tested all dispersal methods - fixed errors in efficiency handling
Created fixtures for integrator and growth
Fixed Growth initialization to avoid error in try statement - still need to refine so KeyError triggered if BOTH kwargs absent
Identified error in GenVeg initialization with plant arrays - need to input a data structure containing arrays (list?)

Pull Request Checklist

Please confirm the following before marking the pull request as "Ready for Review"
(if any of the following items aren't relevant for your contribution please still
tick them so we know you've gone through the checklist):

• The PR is opened in draft mode
• The PR addresses a single feature, fix, or issue
• Code has been linted and is free of style issues (nox -s lint)
• All tests pass locally (pytest, or nox -s test)
• Documentation builds without errors (nox -s docs-build)
• A news fragment
  describing the change has been added
• Relevant docstrings, examples, or changelog entries have been updated
• Related issues or discussions are linked in the description (e.g., Closes #1973)

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