Added a method to get the input layer
Created a test to check getInputLayer method
Corrected getInputLayer method.

Now when you try to get a input layer from an empty MutilayerPerceptron
you will receive a null pointer.
Added commenters to the getInputLayer method
Added getInputLayerSize

This method will return the size of the input layer. If the input layer
does not exist, it will return zero.
Added a test to check getInputLayerSize method
Added commenters to getInputLayerSize method
Added getOutputLayer method
Added a test to check getOutputLayer