Demonstrate a simple linear neural network with bandicoot

This is another proof-of-concept to make sure things are headed in the right direction.

• make sure ensmallen works with bandicoot (#9 (closed))
• implement a multilayer linear feedforward neural network with a simple nonlinearity like a sigmoid for bandicoot outside of mlpack's framework, directly with ensmallen
• demonstrate speedup vs. arma::mat for that simple neural network
• implement the same multilayer linear feedforward neural network with mlpack's framework (which likely needs to be modified somewhat)