Add linear Gaussian model type + validation notebook

[matt-graham]

Adds wrapper type for linear Gaussian models in which filtering can be performed using both Kalman filter (exact) and particle filters (approximate), for the purposes of validating the particle filter implementations. A Pluto notebook validating that the particle filter estimates of the mean and variance parameters of the filtering distributions asymptotically converge to their true values as the ensemble size tends to infinity is also added.

[codecov-commenter]

Codecov Report

Merging #237 (5d5b096) into master (9b39f66) will increase coverage by 0.04%. The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   91.17%   91.22%   +0.04%     
==========================================
  Files           7        7              
  Lines         374      376       +2     
==========================================
+ Hits          341      343       +2     
  Misses         33       33              

Impacted Files	Coverage Δ
src/ParticleDA.jl	89.24% <0.00%> (+0.11%)	:arrow_up:
src/params.jl	85.71% <0.00%> (+2.38%)	:arrow_up:

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[giordano]

What version of MPI.jl do you have in the active environment?

]st -m MPI

After #235 you'll need to do

]resolve
up

in the Pkg REPL to re-resolve and update the environmemt.

[DanGiles]

Yes, that would do it. Thanks Mose. My misunderstanding of how the Project.toml files work. Happy to merge this now @matt-graham, thanks for the updates.