readMNIST(folder)trainData and trainLabels and for the test
data testData and testLabels. To start the
function The files "train-images-idx3-ubyte",
"train-labels-idx1-ubyte', "t10k-images-idx3-ubyte", and
"t10k-labels-idx1-ubyte" have to be in the folder given by
the parameter folder. The folder name must end with
a slash.