layer
integer vector or character value, index (indices) of layer(s)
whose activation function will be changed or character:
"a" denotes all layers, "h" - hidden layer(s), "o" - the output layer
activation
character string, activation function name, admissible
options are: "threshold", "sym_threshold", "linear",
"sigmoid", "sym_sigmoid" (and "tanh"), "sigmoid_approx",
and "sym_sigmoid_approx"
slope
numeric value, activation function slope parameter, if 0
the default parameter value is chosen for each activation function