Mutable NucleicAcidSubstitutionModels

Types of changes

This PR implements the following changes:

[x] :sparkles: New feature
[ ] :bug: Bug fix (A non-breaking change, which fixes an issue).
[ ] :boom: Breaking change (fix or feature that would cause existing functionality to change).

Modifies all the NucleicAcidSubstitutionModels to allow them to be mutable with new function, setrate!.

We need the models to be mutable because we want to use this package in another package that will optimize model rates based on data.

julia>  model = K80(0.5, 1.0);

julia> setrate!(model, [.5, .5])
Kimura 1980 model (absolute rate form)
α = 0.5, β = 0.5

[x] :art: The changes implemented is consistent with the julia style guide.
[x] :blue_book: I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
[x] :blue_book: I have added or updated relevant user and developer manuals/documentation in docs/src/.
[x] :ok: There are unit tests that cover the code changes I have made.
[x] :ok: The unit tests cover my code changes AND they pass.
[ ] :pencil: I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository. I couldn't find this CHANGELOG.md file or a template, but I'd be happy to create one if you point me in the right direction.
[x] :ok: All changes should be compatible with the latest stable version of Julia.
[x] :thought_balloon: I have commented liberally for any complex pieces of internal code.