Add weights for Elman SRN contexts

[thallingstad]

As mentioned by me in issue #10.

This adds ContextWeights and ContextChanges: 3-dimensional arrays of weights and changes, one matrix for each context layer. The weights are calculated just as if they were extra input layers.

The contexts with weights are initialized in SetContexts just as before, so this should not break backward compatibility.

[jonhkr]

Thanks 💙