Neural networks: Implement basic FFNN.
Refactor NN functions to support multiple activation and reduction functions.

Add API for specifying an activation function per layer. Add API for specifying
different reduction functions for network e.g. summation or product reduction.
Refactor NN functions.
Add package for NN object.
Move NN object to correct package.
Rename NN methods for clarity. Add NN test.

Add test for neuron function. A single neuron represents a perceptron.