ml.mlp add monitor output error while training ? #enhancement

[FredVoisin]

Since training a MLP may take quite a long time, it may be usefull to monitor the MLP output error while training, such as a mean error at each epoch loop.
Then, such a monitoring may provide the ability to stop (or continue) the training process at any time by clicking on a message (stop, or train).

[jamiebullock]

This is good suggestion, if it's not a big job I'll add it to the next release.