Standardize UKI constructor to be consistent with EKS, document kwargs

[Zhengyu-Huang]

The UKI constructor modification

function Unscented(
    u0_mean::Vector{FT},
    uu0_cov::Matrix{FT};
    α_reg::FT = 1.0,
    update_freq::IT = 1,
    modified_unscented_transform::Bool = true,
    prior_mean::Union{Vector{FT}, Nothing} = nothing,
) where {FT <: AbstractFloat, IT <: Int}

require α_reg, update_freq, modified_unscented_transform, and prior_mean as keyword arguments
clean up κ and β.

[ilopezgp]

LGTM

[Zhengyu-Huang]

bors r+

[bors]

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