Fixes to FillImputer

[ablaom]

This PR addresses #287 and #286. To this end I have added a UnivariateFillImputer. This shall make refactoring after #288 easier, or might be a model for implementing #288.

To do:

• update model metadata

[OkonSamuel]

@ablaom I know this has already been merged. I hope you don't mind me making a slight change here.

 w = copy(vnew) # transform must be non-mutating
        w[ismissing.(w)] .= filler
        w_tight = convert.(nonmissing(eltype(w)), w)

the above code has too much allocations (w = copy(vnew) and w_tight = convert.(nonmissing(eltype(w)), w). Since a new array must always be created due the requirement that transform must be non-mutating , i feel that the following rewrite is slightly more efficient (Any slight increase in efficiency is required. right?)

w_tight = similar(vnew, nonmissing(eltype(vnew)))
 @inbounds for i in eachindex(vnew)
       ismissing(vnew[i]) ? (w_tight[i] = filler ) : (w_tight[i] = vnew[i])
     end

Maybe the following contrived example can help show why?

using Random, BenchmarkTools
function h1!(vnew, filler) #code avoiding double allocation
       w = similar(vnew, nonmissingtype(eltype(vnew)))
       @inbounds for i in eachindex(vnew)
       ismissing(vnew[i]) ? (w[i] = filler ) : (w[i] = vnew[i])
       end
       w
end
function h2!(vnew, filler) #code with double allocations
       w = copy(vnew) # transform must be non-mutating
               w[ismissing.(w)] .= filler
               w_tight = convert.(nonmissingtype(eltype(w)), w)
       w_tight
end
n = [repeat([missing],10000)..., rand(20000)...]; #array containing missing values
n1 = copy(n)
n2 = copy(n)
shuffle!(n);
n3 = copy(n);
n4 = copy(n);
julia> @btime h1!($n1, 0);
  38.010 μs (2 allocations: 234.45 KiB)
julia> @btime h2!($n2, 0);
  144.883 μs (9 allocations: 584.45 KiB)
julia> @btime h1!($n3, 0);
  166.380 μs (2 allocations: 234.45 KiB)

julia> @btime h2!($n4, 0);
  173.802 μs (9 allocations: 584.45 KiB)

h1! compared to h2! is slightly faster and involves little allocations (this becomes important if this code is called by other functions).

[ablaom]

Yes great idea! Can you make a new PR?

[ablaom]

And thank you for investigating.