Make generate_batches function general by using varargs

[JordiBolibar]

Right now the generate_batches function is more or less hardcoded to a given number of input features. Although not straightforward, since the inputs are a combination of Vectors and scalars, a variable number of input features should be automatically assimilating by using varargs:

function generate_batches(batch_size, θ, UD, features...; shuffle=true)

end