Additions to metrics module.

I've changed the error in metrics, it's now normalised by the number of
training examples. Added two more metrics. vary_data trains the net on
varying sizes of our training data and plots that to see if the amount
of data is an issue. reg_parameter sweeps through a range, varying a
regularisation parameter so that we can find the best one for a net to
correct overfitting.
Removed error completely from multilayer test just to clean up. It now
uses the regularised error function defined in metrics.