update_posterior for FiniteGP itself

[st--]

The low-rank update is implemented for the VFE sparse approximation, but this should of course be just as easy for an exact FiniteGP.:)

[willtebbutt]

I forgot, we essentially have this:

AbstractGPs.jl/src/exact_gpr_posterior.jl

Line 38 in 09fa526

function posterior(fx::FiniteGP{<:PosteriorGP}, y::AbstractVector{<:Real})

If you call posterior on a FiniteGP{<:PosteriorGP}, the appropriate thing will happen :)

[st--]

It's quite confusing that in the exact case, you call posterior, in the approximate case you call update_posterior....

[willtebbutt]

Agreed. I would be in favour of deprecating update_posterior in favour of simply posterior.