Improve show output for AbstractGPs objects

[theogf]

Right now this is disastrous mess, we should agree on what is the essential info and how to print it.
For example this is what the output of a FiniteGP returns:

AbstractGPs.FiniteGP{GP{AbstractGPs.ZeroMean{Float64}, TransformedKernel{SqExponentialKernel{Distances.Euclidean}, ScaleTransform{Float64}}}, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}, Diagonal{Float64, FillArrays.Fill{Float64, 1, Tuple{Base.OneTo{Int64}}}}}(
f: GP{AbstractGPs.ZeroMean{Float64}, TransformedKernel{SqExponentialKernel{Distances.Euclidean}, ScaleTransform{Float64}}}(AbstractGPs.ZeroMean{Float64}(), Squared Exponential Kernel (metric = Distances.Euclidean(0.0))
        - Scale Transform (s = 0.5))
x: -10.0:0.20202020202020202:10.0
Σy: [1.0e-8 0.0 … 0.0 0.0; 0.0 1.0e-8 … 0.0 0.0; … ; 0.0 0.0 … 1.0e-8 0.0; 0.0 0.0 … 0.0 1.0e-8]
)