use column names instead of indices

[tiemvanderdeure]

This package currently uses indices throughout, which can lead to unexpected errors and confusing results. I think we should switch to use keys instead.

For instance

N = 1000
X = (x1 = rand(Float32, N), x2 = randn(Float32, N), x3 = categorical(rand('a':'c', N)))
y = categorical(bitrand(N))

model = MLJFlux.NeuralNetworkBinaryClassifier(epochs = 10, builder=MLJFlux.MLP(; hidden=(5,4)), batch_size = 100)
mach = machine(model, X, y)
fit!(mach)

# this errors
predict(mach, (x3 = X.x3, x1 = X.x1, x2 = X.x2))

# this is false!
all(predict(mach, (x2 = X.x2, x1 = X.x1, x3 = X.x3)) .≈ predict(mach, X))