Use of multiple dispatch and nothing

[MaximeBouton]

I think there are a couple type checks and if statements that could be handled by multiple dispatch as well as some checks for boolean that could use nothing or missing instead.

Examples of lines that could use nothing:
https://github.com/sisl/GaussianFilters.jl/blob/master/src/kf.jl#L57
https://github.com/sisl/GaussianFilters.jl/blob/master/src/kf.jl#L56

Examples of lines that could use multiple dispatch:
https://github.com/sisl/GaussianFilters.jl/blob/master/src/ukf.jl#L27
https://github.com/sisl/GaussianFilters.jl/blob/master/src/ukf.jl#L218
https://github.com/sisl/GaussianFilters.jl/blob/master/src/ukf.jl#L225