Suspicious memory explosions - Add __del__ method to differentiable

Something strange is happening when the graph is dynamically altered that seems to prevent nodes (or graphs) from being garbage collected.

Adding a **del** method to differentiable that removes links to a node from its parents might help solve this.

Start by locating the differentiable implementation and reproducing the memory growth when the graph is dynamically altered. Trace the parent and node links involved in that scenario, then verify that completed graphs or nodes can be garbage collected without breaking differentiation.

Written by the indexing model from the issue text.