cleanup and refactor kernel rules and documentation

[rafaqz]

Description

This PR reorganizes the kernel rule syntax so they are easier to define (again for nice code in the methods paper!) The syntax changes can be seen in the tests, but basically you can do:

InwardsDispersal(radius=2, formulation=ExponentialKernel())

And not create a DispersalKernel manually at all. The syntax is identical for OutwardsDispersal.

It also separates out the formulation and distance methods into separate files so the various components are clearer.

Changes (choose one only, with x or click after creating PR):

• [ ] New formulation/rule
• [ ] Documentation
• [ ] Bug fix for existing rules (non-breaking syntax)
• [x] Minor change to existing rules (no breaks to old syntax)
• [ ] Major change to existing rules (breaking syntax, needs a breaking version bump)

Checklist:

• [x] My code follows the BlueStyle style guide
• [ ] My code is commented with explanations of anything hard to understand
• [x] There are no unnecessary or unrelated code changes included in this PR
• [x] There are no depencecies added: if so, please consider if they are necessary and explain why they are required

For bugfixes

• [ ] My PR addresses a single bug/connected group of bugs
• [ ] I have included a test that will ensure the bug does not recur

For new Rules

• [ ] My PR is limited to a clear theme/cenceptual unit. (Otherwise make multiple PRs that can be discussed separately)
• [ ] New rules have tests in a file of the same name as the rule file, e.g. /test/newrulename.jl
• [ ] I have cited the source of the new formulation/s, if applicable
• [x] I have added a documentation string to formulation struct/s, and added an entry in the docs