CHANGES:
- Implemented an untested version of stochastic gradient descent.
- (BUG FIX) Made the Copy function for Nodes also copy the span values.
- (BUG FIX) Recorded span values for the outcome tree of RecoverFromBackpointer.
- Added a triple outer product function for Armadillo vectors.
- Added training parameters and corresponding getters/setters.
- Made the marginal test do no tensor training.

TODO:
- Do gradient check.