Fix errors relating to RecursiveArrayTools, Base.stack, and GaussianDistributions.jl

[nathanaelbosch]

I believe SciMLBase changed something about how solution indexing and plotting works, and it rougly relies on

indexed_solution = sol(plott; idxs = batch_symbolic_vars) 
push!(tmp, indexed_solution[idxx, :])

This indexing with indexed_solution[idxx, :] failed on our arrays of Gaussians. After a lot of digging, I found that there were multiple issues:

1. Base didn't quite know how to handle the Gaussian type.
2. GaussianDistributions.jl implemented an iterator interface, to allow people to do

   mean, cov = Gaussian(1, 0.1)

   Base.stack did not interact well with this.

I have fixed this by removing GaussianDistributions.jl and instead implementing our own minimal Gaussian type which brings everything we need (and not much more), which could possibly me moved out into a separate SimpleGaussians.jl package in the future. According to #278 this might also fix some more type piracies (not checked yet though).

Left to do:

• clean up the code in src/gaussians.jl 🧹
• run tests 🧪
• review PR 📖
• merge 🚀

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 28 lines in your changes missing coverage. Please review.

Project coverage is 91.84%. Comparing base (0525b2d) to head (eb40e03).
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/gaussians.jl	61.01%	23 Missing ⚠️
src/solution.jl	80.00%	3 Missing ⚠️
src/filtering/update.jl	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
- Coverage   92.92%   91.84%   -1.08%     
==========================================
  Files          44       44              
  Lines        2147     2219      +72     
==========================================
+ Hits         1995     2038      +43     
- Misses        152      181      +29     

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