Optimize computation in distancebased.jl

[thevolatilebit]

• There is no need to compute the variance each time in:
  

  CEEDesigns.jl/src/GenerativeDesigns/distancebased.jl

  Lines 8 to 13 in d1d09c3

  	QuadraticDistance(; λ = 1, standardize = true) =
  	function (x, col; prior = ones(length(col)))
  	σ = standardize ? var(col, Weights(prior); corrected = false) : 1
  	return λ * (x .- col) .^ 2 / σ
  	end

• It might be better to preallocate the vectors in
  

  CEEDesigns.jl/src/GenerativeDesigns/distancebased.jl

  Lines 119 to 128 in d1d09c3

  	vec_evidence = map(colname -> get(evidence, colname, 0), non_targets)
  	distances = map(eachrow(data)) do row
  	vec_row = map(
  	colname -> haskey(evidence, colname) ? row[colname] : 0,
  	non_targets,
  	)
  	z = vec_evidence - vec_row
  	dot(z, Λ * z)
  	end